US 3628503 A
Description (OCR text may contain errors)
1111 States Patent Clemens Neuhaus; Klaus Boehme, both of Erlenbach; Hermann Gutermuth, Kleinwallstadt, all of lnventors Germany Appl. No. 848,038 Filed Aug. 6, 1969 Patented Dec. 21, 1971 Assignee Glanzstofi AG Wuppertal, Germany Priority Aug. 6, 1968 Germany APPARATUS FOR COATING WOVEN 0R NONWQVEN WEBS 3 Claims, 4 Drawing Figs.
lnt. Cl B05c 3/00 lField of Search 1. 1 18/410,
4l3,4l5,414,4l 1,407, 259; 18/13 P, 15 F, 15 R  References Cited UNITED STATES PATENTS 1,667,408 4/1928 Allen 118/415 X 2,796,146 6/1957 Hersey etal.. 118/410 UX 3,083,685 4/1963 Colgan 118/410 3,419,934 1/1969 Lovett 18/15 F X Primary Examiner-John P. Mclntosh Attorney-.lohnston, Root, OKeeffe, Keil, Thompson &
Shurtleff ABSTRACT: The invention consists of an apparatus and process for applying a coating to a woven ornonwovcn web. The apparatus is comprised of a casting box for holding a coating material. an arrangement of slotted, rotating and nonrotating tubes for supplying a moving stream of coating composition into a pressure balancing zone above the web, a means for supplying the coating composition to the tubes, and a doctor blade suitably disposed above the web to achieve a coating of uniform thickness. In the process, the movement of the coating composition relative to the web is in the form of periodically recurring, parallel lines, intersecting the center axis of the web at a constant angle of 0 a 90. By utilizing the apparatus and process disclosed herein, a uniform coating, free of color bands or strips, can be achieved.
SHEU 1 OF 2 INVENTORS: CLEMENS NEUHAUS KLAUS BOEHIVIE HERMANN GUTERMUTH ATT'YS PAIENIEB mm m SHEET 2 OF 2 INVENTORS: CLEMENS NEUHAUS KLAUS BOEHME HERMANN GUTERMUTH APPARATUS lFOllt COATING WOVEN on nonwoven wens BACKGROUND OF THE INVENTION This invention relates to an apparatus for applying a coating to a woven or nonwoven web and more particularly to an apparatus for applying a uniform coating free of color strips.
There are a number of existing processes for coating woven or nonwoven webs. The choice of a particular process depends upon the consistency of the coating composition, on the thickness of the application, and on the type of carrier material. Thus, for example, thin solutions can be applied by the reverse-roll-coat process; while thick solutions, pastes or gels, are generally applied by the brushing (spreading) process. The spreading process is the more complex one having a greater degree of variation. Coating compositions, including pastes and gels, can be combined with color pigments or be dyed using soluble dye materials to achieve a desired effect. The spreading process, which includes the application of coating materials by means of roller wipers, rubber blanket wipers, or support wipers, can be used to achieve a coating of uniform thickness, but it is frequently difficult, using this process, to obtain a uniform color over the entire width of the material. In the conventional spreading processes, longitudinal stripes form and become visible because of the more or less great differences in color intensity. So far, no satisfactory explanation has been given for the occurrence of these variations in color. One example of a coating composition which is difficult to handle and produces the aforementioned color stripe, is polyurethane gel. The polyurethane solid content lies, as a rule, between and 40 percent by weight.
One of the conventional methods for applying a polyurethane gel coating consists of supplying the gel over a stationary feed device in front of a doctor blade. In this process, however, a rotating bulge of gel material occurs in front of the doctor blade during the spreading process. After passing the doctor blade, the web has, as a rule, a smooth but nonhomogeneously colored covering layer which is permeated with longitudinal stripes. In addition, air is often imbedded by the rotating bulge thereby producing a faulty covering layer.
The streakiness in color cannot be avoided either by reducing or by enlarging the bulge. Nor is it possible in the conventional process to achieve any substantial improvement of the covering layer by applying the gel through a stationary distributor tube situated in the rotating bulge. Similar attempts to apply the coating composition through a tube with a longitudinal slit or with a row of holes have likewise failed to achieve the desired results.
Other attempts to limit or reduce the bulge by utilizing a casting box installed on the doctor blade, into which the gel is supplied at different places, have failed to achieve a covering layer free of stripes. This approach has shown, however, that by engaging a casting box in front of the doctor blade, it is possible to obtain an improvement in the smoothness of the covering layer because of the absence of air bubbles.
Finally, attempts have been made to mount a traverse device above the casting box ahead of the doctor blade to convey the gel downwardly through a tube into the casting box. This process has proven to be of little advantage, since the reciprocating back and forth of the tube causes air bubbles in the gel resulting in nonuniform coating layers.
SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved coating apparatus for applying a stripe-free colored covering layer on woven or nonwoven webs.
It is a further object of the present invention to provide coating apparatus which will prevent the occurrence of air bubbles in the coating composition thereby achieving a more uniform coating layer.
In a preferred embodiment of the invention, a woven or nonwoven web is arranged to pass over a roller means. A wiper or doctor blade is disposed above the moving web. A
casting box is arranged in front of the doctor blade and above the moving web such that the casting box and the doctor blade form an enclosed vessel for holding a quantity of coating material. A double tube, having an inner tube and an outer tube, is mounted in the casting box above the moving web. An outer tube having a straight slot facing the roller is attached in a fixed nonrotating position in the casting box. The straight slot extends the full width of the casting box. The inner tube is rotatably mounted in the outer tube and is connected by a shaft means to a source of driving power. A spiral slot, which also extends the full width of the casting box, encircles the inner tube and makes at least one turn around the circumference of the inner tube. In the preferred embodiment of the invention, the spiral slot in the inner tube covers the width of the casting box in two turns. The inner space of the inner tube is connected to a source container for the coating composition.
Since the inner tube contains a spiral slot which makes at least one turn around the circumference of the inner tube, and since the outer tube has a straight slot, there will be one or more places along the length of the tubes where the spiral slot and straight slot will overlap. The points of overlap are called feed places since the coating composition can pass from the space interior to the inner tube through the overlapped slots into the vessel formed by the casting box and the doctor blade.
The use of the double tube as a rotating gel distributor makes possible the construction of a compact, closed casting box in which the static pressure in the gel can be increased by introducing an inert gas, under pressure, into the casting box above the coating composition.
Inside the casting box, the space lying between the feed places and the moving web is designated as a pressure balancing zone. The pressure balancing zone is utilized to diffuse the orifice pressure in the coating composition at the feed places to avoid any streakiness which might be caused by pressure variations. In the case of polyurethane gels, the spacing between the outer tube and the roller lies generally between 0.5 centimeters and 5 centimeters. With this spacing, a sufficiently large pressure balancing zone is created. Depending upon the viscosity conditions, however, this spacing can be larger or smaller.
During the coating process, the uncoated web is conveyed and supported on a roller and passed beneath the casting box whereupon the coating composition is applied to the web. After passing through the pressure balancing zone, the coating composition is wiped by the doctor blade onto the web in a known manner. The coated web emerges under the doctor blade, the height of which is suitably adjusted to achieve a coating ofuniform thickness.
The rotation of the inner tube causes the overlap (feed place) of the spiral slot and straight slot to move from one side of the casting box to the other. This movement of the feed place relative to the movement of the web may be graphically depicted as a series of recurrent parallel lines (relative movement lines) intersecting the central axis of the web at an angle (I, wherein 0 a Since the relative movement of the feed places with respect to the moving web is not a zigzag movement, as in the conventional processes using traverse motion feed tubes, it has been possible to achieve a stripe-free coating layer. In order to achieve this stripe-free coating, it appears essential that the angle a be greater than 0 but less than 90, whereupon the angle between the relative movement lines and the doctor blade is an acute angle which always remains constant. Under these circumstances, any occurrence of obliquely running stripes in front of the doctor blade will be eliminated and the web leaving the doctor blade will have a stripe-free covering layer.
Preferably, the tangent. angle of a lies between 1 and 8. This means that the angle between the relative movement lines and the central axis of the web lies between 45 and 82. In this range, the spacings between the parallel relative movement lines are relatively small and the resolving capacity of the doctor blade is especially good. Best results appear to be obtained when the tangent of the angle a lies between 3 and 4; i.e., when the angle lies between 72 and 76.
In choosing the number of turns of the spiral slot formed on the inner tube, it should be noted that the angle a is dependent upon the velocity with which the web passes through the apparatus, on the rotational speed of the inner tube, on the width of the endless web, and on the number of turns of the spiral slot on the inner tube. If the width of the web and its velocity are held constant, the stripes in the covering layer will be avoided with equal effect if the quotient of the turning speed of the inner tube to the number of turns of the spiral slot on the inner tube is constant. As previously stated, the inner tube is provided with at least one slot covering, in spiral form, the entire casting box width in at least one turn. Of course, it is also possible to use multicourse slots, i.e., parallel slots. If multicourse spiral slots are employed, then it is theoretically no longer necessary that the number of turns of the spiral slot on the inner tube has to be an integral number greater than or equal to one. It is expedient, however, in order to avoid overlapping of the zones covered, to utilize an integral number of turns of the spiral slot on the inner tube and, in the case of multicourse slots, to keep the number of turns equal.
The preferred embodiment of the invention described herein has substantial advantages over the known or conventional coating machines. In particular, the absence of a reciprocating traverse and the presence of a casting box brings about the following important advantages:
First, a reduction of the boundary layer between air and coating composition is achieved; second, it is possible to achieve a coating layer which has few faults; and third, it is possible to subject the coating composition to a constant static pressure so that constant conditions of application can be achieved.
Further features and advantages of the invention will become more readily apparent from the following detailed description of a specific embodiment of the invention when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS fig 1 is a side elevation, partially in section, of one embodiment of the invention;
FIG. 2 is a section view taken along line 2-2 in FIG. 1;
FIG. 3 is a schematic representation of a web passing through the gel distributor embodied in the present invention showing the time and spatial relationships between the gel feed places and the moving web; and
FIG. 4 is a schematic representation showing the time and spatial relationship between a gel feed tube in traverse motion and a moving web.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, casting box 1 is arranged in front of wiper or doctor blade 2 and immediately above roller 5. Outer tube 3 is rigidly and nonrotatably mounted as shown in casting box 1. Slot 4, which extends approximately the width of casting box 1 and is situated as shown in tube 3, is oriented directly above and facing roller 5. Sleeves 11, seal off tube 3 as it passes through casting box 1. Inner tube 6 which is rotatably mounted in outer tube 3, is connected through shaft 14 to a drive means (not shown). Spiral slot 7, situated as shown on inner tube 6, extends the width of casting box 1 and is arranged to make at least one turn around inner tube 6'over the width of casting box I.
As indicated herein, slots 4 and 7 extend the width of casting box 1. This is intended to mean that normally the length of these slots can be identical with the width of casting box 1. It is, of course, possible to make the slots somewhat shorter than the casting box width if that is desired. The width of web 12 on the other hand is normally somewhat greater than the length of slot 4. In the case of the apparatus represented in FIG. 1, spiral slot 7 forms two turns over the width of casting box I. As used herein, a turn is defined as a spiral curve which encloses the circumference of inner tube 6, and therefore, in axial projection, includes one rotation through an angle of 360.
A viscous coating composition 15 is pumped from container 27 through tube 25 into gel tube 9 in the direction designated by the arrow A shown in FIG. 1. The ends of tubes 3 and 6 extend as shown beyond casting box 1. Viscous coating composition 15 is conveyed from gel tube 9 through annular space 16 in outer tube 3 and slot opening 17 inner tube 6 into inner space 8 of tube 6. The places where slots 4 and 7 cover each other are designated feed places 23. Coating composition 15 is delivered from inner space 8 through the feed places 23 into pressure balancing zone 19 which lies between outer tube 3 and roller 5.
Uncoated web 12, moving in the direction indicated by arrow C" in FIG. 2, passes between casting box I and roller 5. During operation of the invention, casting box 1 is partially filled with coating composition as shown in FIG. 2. Casting box 1 is disposed above the uncoated web 12 such that none of the coating composition 15 wells forth under the side of casting box 1 lying opposite wiper or doctor blade 2. With casting box 1 so disposed, no rotating bulge of coating composition 15 will form in the area 18 between casting box 1 and uncoated web 12. As the uncoated web 12 passes through the pressure balancing zone 19, coating composition 15 is applied, and coated web 13 emerges under wiper or doctor blade 2 in the direction indicated by arrow D in FIG. 2. Doctor blade 2, which performs a wiping operation, is arranged above coated web 13, so as to obtain a uniform coating thickness.
In order to achieve pressure balancing and to generate sufficiently great static pressure, an inert gas is introduced through connection 10 in the direction indicated by arrow B." This gas acts against the surface 26 of coating composition 15 to increase the pressure in the coating composition.
FIG. 3 is a schematic representation showing the relative movement between feed places 23 and the centerline of moving coated web 13. This relative movement can be represented by a series of parallel straight lines 20 which intersect the centerline 24 of web 13 at a constant angle a where 0 a 90. Of the three feed places 23 shown in FIG. 3 at least two may be utilized to supply coating composition 15 into the pressure balancing zone 19. Once during each revolution of inner tube 6, however, three feed places can be utilized namely, at both ends and in the middle of slot 4.
FIG. 4 is a schematic representation, included for comparison purposes with FIG. 3, showing the relative movement, in a conventional wiper machine, between a feed place 22 in traverse motion and a web 13. This relative movement can be represented by a zigzag line 21 which passes the center line 24 alternately at an angle B less than 90 and a greater than 90.
EXAMPLE A nonwoven carrier material (web), centimeters in width, is supplied to the apparatus described in detail in FIGS. 1 and 2, at a velocity of 2 meters per minute. Inner tube 6 has a two-turn spiral slot 7. The feed places 23 will have a separation of 40 centimeters.
The widths of slots 4 and 7 are 3 millimeters. The inner tube 6 rotates at 10 rpm. Consequently, I revolution requires 0.1 minutes (6 seconds). During this 6 second period, feed places 23 will move along straight slot 4 by 40 centimeters. During the same interval, 20 centimeters of web 13 move past slot 4.
Accordingly, the tangent of the angle a between the line 20, which represents the relative movement between feed places 23 and web 13, and the centerline 24 of web 13, is two. The angle a is, therefore, about 64. Under these conditions, a covering layer consisting of a coating composition 15 made up of a polyurethane gel having a solid substance content of 20 percent by weight, will be completely free of stripes.
While the foregoing description has referred to a specific embodiment of the invention, it will be apparent that many modifications and variations may be made therein without departing from the spirit and scope of the invention.
We claim: 1. Apparatus for applying a stripe-free coating to a woven or nonwoven web comprising in combination:
a. a roller means disposed below said web for conveying and supporting said web;
b. wiper means disposed above and substantially perpendicular to the centerline of said web;
c. casting box means arranged forward of said wiper means;
d. a first tube means rigidly mounted in said casting box means, said first tube means arranged generally above said web;
. a straight slot in said first tube means, said slot being oriented downwardly toward said web and extending the full width of said casting box;
l. a second tube means rotatably disposed in said first tube means, said second tube means extending the full width of said casting box;
g. at least one spiral slot situated in the circumference of said second tube means and extending substantially the full width of said casting box, said spiral slot executing at least one complete turn around the circumference of said second tube means;
h. a cylindrical inner space disposed in said second tube means;
i. a container for storing said coating composition;
j. means for conveying said coating composition from said container to said cylindrical inner space;
k. shaft means operatively associated with said second tube means; and
i. drive means connected to said shaft means for rotating said second tube means.
2. Apparatus in accordance with claim 1 comprising in addia. cover plate means engaging the upper edges of said casting box means;
b. a source of pressurized inert gas; and
c. means for conveying said gas into said casting box means at a point above the surface of the coating composition contained in said casting box.
3. Apparatus in accordance with claim ll wherein said means for conveying said coating composition from said container to said cylindrical inner space comprises in addition: