Patent Grant
6878408
This invention relates to devices and methods for coating substrates and for improving the uniformity of non-uniform or defective coatings.
There are many known methods and devices for coating a moving web and other fixed or moving substrates, and for smoothing the resulting coating. Several are described in Booth, G. L., “The Coating Machine”, Pulp and Paper Manufacture, Vol. 8, Coating, Converting and Processes, pp 76-87 (Third Edition, 1990) and in Booth, G. L., Evolution of Coating, Vol. 1 (Gorham International Inc.). For example, gravure roll coaters (see, e.g. U.S. Pat. No. 5,620,514) can provide relatively thin coatings at relatively high run rates. Attainment of a desired specific average caliper usually requires several trials with gravure rolls of different patterns. Runtime factors such as variations in doctor blade pressure, coating speed, temperature, or liquid viscosity can cause overall coating weight variation and uneven localized caliper in the machine or transverse directions.
Barmarks and chatter marks are bands of light or heavy coating extending across the web. These are regarded as defects, and can be caused by factors such as vibration, flow pulsation, web speed oscillation, gap variation and roll drive oscillation. Chatter marks are commonly repeating, but barmarks can occur as the result of random system upsets. Gutoff and Cohen, Coating and Drying Defects (John Wiley & Sons, New York, 1995) discusses many of the sources of cross web marks and emphasizes their removal by identifying and eliminating the fundamental cause. This approach can require substantial time and effort.
Under some gravure roll coating run conditions, a gravure roll pattern appears in the wet coating. Gravure roll marks can be removed with an arcuate flexible smoothing film located down web from the gravure roll (see, e.g., U.S. Pat. No. 5,447,747); with a smoothing roll or rolls bearing against an intermediate coating roll (see, e.g., U.S. Pat. No. 4,378,390) or with a set of smoothing rolls located down web from the gravure roll (see, e.g., U.S. Pat. No. 4,267,215).
Very thin coatings (e.g., about 0.1 to about 5 micrometers) can be obtained on gravure roll coaters by diluting the coating formulation with a solvent. Solvents are objectionable for health, safety, environmental and cost reasons.
Multiroll coaters (see, e.g., U.S. Pat. Nos. 2,105,488; 2,105,981; 3,018,757; 4569,864 and 5,536,314) can also be used to provide thin coatings. Multiroll coaters are shown by Booth and are reviewed in Benjamin, D. F., Anderson, T. J. and Scriven, L. E. “Multiple Roll Systems: Steady-State Operation”, AIChE J., V41, p. 1045 (1995); and Benjamin, D. F., Anderson, T. J. and Scriven, L. E., “Multiple Roll Systems: Residence Times and Dynamic Response”, AIChE J., V41, p. 2198 (1995). Commercially available forward-roll transfer coaters typically use a series of three to seven counter rotating rolls to transfer a coating liquid from a reservoir to a web via the rolls. These coaters can apply silicone release liner coatings at wet coating thickness as thin as about 0.5 to about 2 micrometers. The desired coating caliper and quality are obtained by artfully setting roll gaps, roll speed ratios and nipping pressures. Another type of coating device that could be described as a multiroll coater is shown in U.S. Pat. No. 4,569,864, which describes a coating device in which a thick, continuous premetered coating is applied by an extrusion nozzle to a first rotating roll and then transferred by one or more additional rolls to a faster moving web.
Some of the above-mentioned coating devices employ a series of smoothing brushes that contact the applied wet coating on a web and help to reduce coating irregularities. According to page 76 of the Booth article entitled “The Coating Machine”, from 4 to 10 smoothing brushes were used in early coating machines. Smoothing brushes smear the coating under the brush, but do not contact and then re-contact the wet coating.
Rolls have sometimes also been used for smoothing. Usually these are counter-rotating rolls whose direction of motion is opposite that of a moving web. Page 77 of the Booth article shows a squeeze roll coater equipped with four “reverse running” (counter rotating) smoothing rolls located down web from an applicator roll. Examples 1-7 and 10 of U.S. Pat. No. 4,267,215 patent show the application of a continuous coating to a plastic film wherein the wet coating is contacted by an undriven corotating stabilizing roll 68 (whose direction of motion in the contact zone is the same as that of the moving plastic film) and a set of three equal diameter counter rotating spreading rolls 70. The respective diameters of the stabilizing roll and spreading rolls are not disclosed but appear from the Drawing to stand in a 2:1 ratio. In Example 10 of the '215 patent, the applicator roll speed was increased until the uniformity of the coating applied to the web began to deteriorate (at a peripheral applicator roll speed of 0.51 m/s) and surplus coating liquid began to accumulate on the web surface upstream of the rolls 70 (at a peripheral applicator roll speed of 0.61 m/s). Coatings having thicknesses down to 1.84 micrometers were reported. Coating devices employing smoothing rolls such as those described above could contact and then re-contact the wet coating on a moving web, but only a relatively small number (e.g., four or less) of such rolls appear to have been employed.
During continuous web coating operations, unintended surges in coating caliper sometimes occur. Surges can arise from a variety of causes including operator error, system control failures, machinery failures and increases in the supply (or reductions in the viscosity) of the coating liquid. This can lead to a temporary large increase in coating caliper (e.g., by a factor of 2 or even 10 or more). One typical example is a momentary loss of the hydraulic pressure that holds closed the metering gap of a reverse roll coater. Unless the drying section of a coating process line is designed with significant overcapacity, the occurrence of such a surge can cause wet web to be wound up at the end of the process line. This can make the entire wound roll unusable. In addition, if the coating liquid contains a flammable solvent, then flammable concentrations of solvent paper can arise at the winder. Since the roll winding station often causes substantial static electrical discharges, fires or explosions can occur.
Occasionally an unintended gross deficiency in coating caliper will occur during a continuous web coating operation. Defects of this nature can arise from a variety of causes including operator error, air entrainment, system control failures, machinery failures, interruptions in the supply (or sudden increases in the viscosity) of the coating liquid, and changeover of the web or coating roll. This can cause significant portions of a web to be uncoated and can generate undesirable scrap.
The improvement brushes and smoothing roll devices described above generally are not able to compensate adequately for gross coating defects such as a substantial coating caliper surge or a complete absence of coating over a significant portion of a web.
In the above-mentioned U.S. Pat. No. 6,737,113 B1, repeating and random coating defects are eliminated or at least significantly reduced through the use of pick-and-place contacting devices. Rotating rolls (and especially undriven rolls that can corotate with the substrate as it passes by the rolls) are a preferred type of pick-and-place device in the patent. Rolls having periods of contact (defined as the time between successive contacts by a point on the device with the substrate) that were equal to one another were not preferred. Instead, the preferred pick-and-place devices were differently sized rolls, or rolls operated at different speeds, with the sizes or speeds (and thus the periods of contact) not being periodically related to one another.
The present invention provides, in one aspect, coating devices and methods using a number of pick-and-place devices (e.g., rolls) whose periods of contact with a substrate are equal or substantially equal to one another. The devices can be ordered in standard sizes commonly stocked by suppliers (e.g., roll suppliers). The purchase and installation of standard size devices is inexpensive and more readily accomplished than the purchase and installation of special size devices. The use of a sufficiently large number of such pick-and-place devices facilitates the formation of continuous void-free uniform coatings despite the occurrence of unintended coating caliper surges, depressions or voids. Thus the invention provides, in one aspect, a method for improving the uniformity of a wet coating on a substrate comprising contacting and re-contacting the coating with wetted surface portions of a sufficient number of periodic pick-and-place devices having the same or substantially the same periods of contact with the substrate so that coating caliper defects ranging from a complete absence of coating to an excess of as much as 200% of the average coating caliper are converted to range from 85% to 115% of the average coating caliper.
In another aspect, the invention provides a method for improving the uniformity of a wet coating on a substrate comprising contacting and re-contacting the coating with wetted surface portions of at least five periodic pick-and-place devices having the same or substantially the same periods of contact with the substrate.
When all the pick-and-place devices have the same period of contact, the invention enables a reduction in the magnitude of random coating caliper surges or voids. When the pick-and-place devices have at least a small variation or variations in their periods of contact or when at least one other pick-and-place device having a substantially different period of contact (e.g., a period that differs by more than 1% from the average period of the other devices) is employed, the invention also enables a reduction in the magnitude of repeating coating caliper surges, depressions or voids.
In another aspect, the invention provides a method for coating a moving web comprising applying thereon a wet coating having a caliper variation and contacting and re-contacting the wet coating with wetted surface portions of one or more rolls having a period of contact with the web, wherein the period of the caliper variation, the size of the caliper variation or the periods of contact of the rolls are changed (e.g., selected or adjusted) to reduce or minimize coating defects.
In another aspect, the invention provides devices for performing the methods of the invention. In one aspect, the devices of the invention comprise an improvement station comprising a plurality of pick-and-place devices that can periodically contact and re-contact a wet coating at different positions on a substrate, wherein the coating has defects and an average coating caliper and wherein the number of pick-and-place devices having the same or substantially the same periods of contact with the substrate is sufficient so that coating caliper defects ranging from a complete absence of coating to an excess of as much as 200% of the average coating caliper are converted to range from 85% to 115% of the average coating caliper. In another aspect, the devices of the invention comprise an improvement station comprising at least five pick-and-place devices that can periodically contact and re-contact a wet coating at different positions on a substrate and have the same or substantially the same periods of contact with the substrate.
In another aspect, the devices of the invention comprise a coating apparatus comprising a coating station that applies an uneven (and preferably discontinuous) coating to a substrate and an improvement station comprising one or more pick-and-place devices that can periodically contact and re-contact the applied coating at different positions on the substrate, wherein the number of pick-and-place devices having the same or substantially the same periods of contact with the substrate is sufficient so that coating caliper defects ranging from a complete absence of coating to an excess of as much as 200% of the average coating caliper are converted to range from 85% to 115% of the average coating caliper. In yet another aspect, the devices of the invention comprise a coating apparatus comprising a coating station that applies an uneven (and preferably discontinuous) coating to a substrate and an improvement station comprising at least five pick-and-place devices that can periodically contact and re-contact the applied coating at different positions on the substrate and have the same or substantially the same periods of contact with the substrate.
In a particularly preferred aspect of the above-mentioned devices, the applied coating has a periodic caliper variation and the period of the caliper variation, the size of the caliper variation or the period of contact of one or more of the devices is changeable (e.g., selectable or adjustable) to reduce or minimize coating defects.
In yet a further aspect, the coating apparatus further comprises a transfer station for transferring the coating from the first substrate to a second substrate.
In yet a further aspect, the coating apparatus further comprises a drying station.
a though 10d are patterned contour plots of coating caliper vs. web distance when a single severe void passes through an improvement station containing 250 equally-sized rolls each having a period of 10 dimensionless web length elements.
e through 10g are line plots illustrating the down web caliper profile as the void of
a through 14n are improvement diagrams illustrating the relationship between dimensionless roll size, dimensionless stripe width and the minimum caliper that can be obtained by periodically applying cross-web coating stripes to a moving web and passing the coated web through an improvement station containing one or more rolls.
a through
FIG. 20 through
Referring to
A type of pick-and-place device 15 that can be used in the present invention to improve a coating on a moving web 10 is shown in FIG. 2. Device 15 has a hub 20 to permit device 15 to rotate about a central axis 21. The hub 20 and axis 21 extend across the coated width of the moving web 10, which is transported past hub 20 on roll 22. Extending from hub 20 are two radial arms 23 and 24 to which are attached pick-and-place surfaces 25 and 26. Surfaces 25 and 26 are curved to produce a singular circular arc in space when surfaces 25 and 26 are rotated about axis 21. Because of their rotation and spatial relation to the web 10, pick-and-place surfaces 25 and 26 periodically contact web 10 opposite roll 22. Wet coating (not shown in
The period of a pick-and-place device can be expressed in terms of the time required for the device to pick up a portion of wet coating from one position along a substrate and then lay it down on another position, or by the distance along the substrate between two consecutive contacts by a surface portion of the device. For example, if the device shown in
The period of a pick-and-place device can be altered in many ways. For example, the period can be altered by changing the diameter of a rotating device; by changing the speed of a rotating or oscillating device; by repeatedly (e.g., continuously) translating the device along the length of the substrate (e.g., up web or down web) with respect to its initial spatial position as seen by a fixed observer; or by changing the translational speed of the substrate relative to the speed of rotation of a rotating device. The periods of individual devices do not need to remain constant over time, and if varied do not need to vary according to a smoothly varying function.
Many different mechanisms can produce a periodic contact with the liquid coated substrate, and many different shapes and configurations can be used to form the pick-and-place devices. For example, a reciprocating mechanism (e.g., one that moves up and down) can be used to cause the coating-wetted surfaces of a pick-and-place device to oscillate into and out of contact with the substrate. Preferably the pick-and-place devices rotate, as it is easy to impart a rotational motion to the devices and to support the devices using bearings or other suitable carriers that are relatively resistant to mechanical wear.
Although the pick-and-place device shown in
The invention is especially useful for, but not limited to, coating moving endless webs and belts. For brevity and unless the context requires otherwise, such a moving endless web or belt will be collectively referred to herein as a “web”. The web can be previously uncoated, or can bear a previously-applied hardened coating, or can bear a previously-applied and unhardened wet coating. Rotating pick-and-place devices are preferred for improving coating quality or minimizing coating defects on such webs. The devices can translate (e.g., rotate) at the same peripheral speed as the moving web, or at a lesser or greater speed. If desired, the devices can rotate in a direction opposite to that of the moving web. Preferably, the rotating pick-and-place devices have the same direction of rotation. More preferably, for applications involving the improvement of a coating on a substrate having a direction of motion, the direction of rotation of at least two such pick-and-place devices is the same as the direction of substrate motion. Most preferably, such pick-and-place devices rotate in the same direction as and at substantially the same speed as the substrate. This can conveniently be accomplished by using corotating undriven rolls that bear against the substrate and are carried with the substrate in its motion.
When initially contacting the coating with a pick-and-place device like that shown in
There is no guarantee that the liquid split ratio between the web and the surface will remain always at a constant value. Many factors can influence the split ratio, but these factors tend to be unpredictable. If the split ratio changes abruptly, a repeating down web caliper variation will result even if the pick-and-place device has been running for a long time. If foreign material lodges on a transfer surface of the pick-and-place device, the device may create a repeating down web defect at each contact. Thus use of only a single pick-and-place device can potentially create large lengths of scrap material.
The invention employs a sufficient number of pick-and-place devices having the same or substantially the same period of contact in order to achieve a desired degree of coating uniformity. The desired degree and thus the preferred number of devices will depend on the intended use of the coated substrate and the nature of the applied coating. Preferably, five or more pick-and-place devices having the same or substantially the same period of contact are used. More preferably, six or more, eight or more, ten or more, twenty or more or even 40 or more such devices are employed.
When coating a moving web, the pick-and-place devices can be arranged down web from a coating station in an array that will be referred to as an “improvement station.” After the coating liquid on the pick-and-place transfer surfaces has built to an equilibrium value, a random high or low coating caliper spike may pass through the station. When this happens, and if the defect is contacted, then the periodic contacting of the web by a single pick-and-place device, or by an array of only a few pick-and-place devices having the same contact period, will repropagate a repeating down web defect in the caliper. Again scrap will be generated and those skilled in coating would avoid such an apparatus. It is in general much better to have just one defect in a coated web rather than a length of web containing multiple images of the original defect.
A random severe initial defect (e.g., a large coating surge, or a complete absence of coating) can be significantly diminished by an improvement station of the invention. The input defects can be diminished to such an extent that they are no longer objectionable. By using the methods and devices of the invention, a new down web coating profile can be created at the exit from the improvement station. That is, by using multiple pick-and-place devices, the multiple defect images that are propagated and repropagated by the first device are modified by additional multiple defect images that are propagated and repropagated by the second and subsequent devices. This can occur in a constructively and destructively additive manner so that the net result is a more uniform caliper or a controlled caliper variation. In effect, multiple waveforms are added together in a manner so that the constructive and destructive addition of each waveform combines to produce a desired degree of uniformity. Viewed somewhat differently, when a coating upset passes through the improvement station a portion of the coating from the high spots is in effect picked off and placed back down in the low spots.
Mathematical modeling of the improvement process of the invention is helpful in gaining insight and understanding. The modeling is based on fluid dynamics, and provides good agreement to observable results.
Similar coating improvement results are obtained when the random defect is a depression (e.g., an uncoated void) or bar mark rather than a spike. The graphs have a similar but inverted appearance and the caliper change is negative rather than positive.
The random spike and depression defects discussed above are one general class of defect that may be presented to the improvement station. The second important class of defect is a repeating defect. Of course, in manufacturing coating facilities it is common to have both classes occurring simultaneously. If a repeating train of high or low coating spikes or depressions is present on a continuously running web, the coating equipment operators usually seek the cause of the defect and try to eliminate it. A single periodic pick-and-place device as illustrated in
Referring for the moment to pick-and place roll 74, the liquid coating splits at lift off point 99. A portion of the coating travels onward with the web and the remainder travels with roll 74 as it rotates away from lift off point 99. Variations in coating caliper just prior to lift off point 99 are mirrored in both the liquid caliper on web 72 and the liquid caliper on the surface of roll 74 as web 72 and roll 74 leave lift off point 99. After the coating on web 72 first contacts roll 74 and roll 74 has made one revolution, the liquid on roll 74 and incoming liquid on web 72 meet at the initial contact point 98, thereby forming a liquid filled nip region 100 between points 98 and 99. Region 100 is without air entrainment. To a fixed observer, the flow rate of the liquid entering this nip contact region 100 is the sum of the liquid entering on the web 72 and the liquid entering on the roll 74. The net action of roll 74 is to pick material from web 72 at one position and place a portion of the material down again at another position.
In a similar fashion, the liquid coating splits at lift off points on the pick-and-place contactor rolls throughout the remainder of improvement station 71. A portion of this split coating re-contacts web 72 and is reapplied thereto at contact points throughout the remainder of station 71.
As with the trains of intermittent pick-and-place contacting devices discussed above, random or repeating variations in the liquid coating caliper on the incoming web will be reduced in severity and desirably the variations will be substantially eliminated by the pick-and-place action of the periodic contacting rolls.
However, by using a suitably large number of devices, the quality of even a grossly non-uniform input coating can be improved. The simulation shown in
The dimensionless caliper or caliper range is plotted in
The white regions 101 and 102 in
e through 10g further illustrate the down web caliper profile as the void of
e shows the initial caliper (plot 108) before and the down web caliper profile after the first 400 web elements pass the first roll (plot 110), second roll (plot 112) and third roll (plot 114). After the third roll, the initial 5 element long void has propagated as five images 114, 116, 118, 120 and 122 having a caliper less than 90% of the average void-free caliper, with images 116, 118 and 120 having a caliper less than 85% of the average void-free caliper.
f shows the profile after passing the fourth roll (plot 124), fifth roll (plot 126) and sixth roll (plot 128). After the sixth roll the initial void is still mirrored as four images 130, 132, 134 and 136 having calipers less than 90% of the average void-free caliper, but with no images having a caliper less than 85% of the average void-free caliper.
g shows the profile after passing the seventh roll (plot 138), eighth roll (plot 140) and ninth roll (plot 142). After nine rolls, all images of the initial void have calipers greater than 90% of the average void-free caliper. Thus in this fashion an initial severe defect has been greatly reduced in severity, thereby permitting recovery of miscoated web that would otherwise have to be scrapped.
Comparable results are found for coating defects characterized by coating excesses rather than voids. For example, if a coating surge results in an initial dimensionless caliper of 2.0 (200% of the average void-free caliper), then use of an improvement station having a sufficient number of rolls as described above can provide coated web in which images of the defect are less than 115% (using six rolls) or less than 110% (using nine rolls) of the average void-free caliper. Thus a web having instantaneous coating caliper defects ranging from a void of 0% to an excess of 200% of a desired or target average caliper value can be converted using a six roll improvement station of the invention into a web whose coating caliper is between 85% and 115% of the desired average caliper value. For coatings of modest uniformity requirements, variations of 85 to 115 percent of the target can be adequately functional. Methods that achieve this degree of uniformity represent a preferred aspect of the invention. In the same fashion, a web having instantaneous coating caliper defects ranging from 0% to 200% of the desired average caliper value can be converted using a nine roll improvement station of the invention into a web whose coating caliper is between 90% and 110% of the desired average caliper value. Methods that achieve this degree of uniformity represent a more preferred aspect of the invention. The invention is of course not limited to use with coatings having the above-mentioned ranges of coating defects. The coating defects can span a smaller or greater overall range. However, examination of the manner in which wet coating defects ranging from a specified minimum value to a specified maximum value are affected by the pick-and-place devices serves as a useful metric for characterizing the nature of the improvement provided by the present invention.
Factors such as drying, curing, gellation, crystallization or a phase change occurring with the passage of time can impose limitations on the number of rolls employed. If the coating liquid contains a volatile component, the time necessary to translate through many rolls may allow drying to proceed to the extent that the liquid may solidify. Drying is actually accelerated by the present invention, providing certain advantages discussed in more detail below. In any event, if a coating phase change occurs on the rolls for any reason during operation of the improvement station, this will usually lead to disruptions and patterns in the coating on the web. Therefore, in general it is preferred to produce the desired degree of coating uniformity using as few rolls as possible. However, under the right conditions very large numbers of rolls (e.g., as many as 10, 20, 50, 100 or even 1000 or more rolls) can be employed in the invention. Drying can be discouraged by placing the improvement station (and optionally the coating station and drying station, if employed) of the coating apparatus in a suitable enclosure and flooding the inside of the enclosure with vapors of any solvents present in the coating liquid. A preferred technique for discouraging such drying is to circulate a non-reactive gas saturated with such vapors through the enclosure as described, for example, in U.S. Pat. No. 6,117,237.
By using multiple pick-and-place rolls, it is possible simultaneously to reduce the amplitude of and to merge successive spikes or depressions together to form a continuously slightly varying but spike- and depression-free coating of good uniformity. As shown above, this can be accomplished by using roll devices of equal diameters that are undriven and corotate with the web at equal speeds. Improvements in coating uniformity can also be obtained by varying the diameters of a train of roll devices. If the rolls are not rotated by the traction with the web, but instead are independently driven, then the period of each roll is related to its diameter and rate of rotation.
The desired caliper will of course depend on the particular application. For example, the requirements for coated abrasives, tape and optical films will all differ from one another. The requirements will also differ within a class of products. For example, coarse abrasives used for woodworking have a less stringent caliper uniformity requirement than microabrasives used for polishing disk drive parts. In general, the thinner the average caliper, the more stringent the uniformity requirement.
As belt 170 circulates past the pick-and-place rolls 172, 174, 176, 178, 180 and 182, the coating liquid on belt 170 contacts the surfaces of pick-and-place rolls 172, 174, 176, 178, 180 and 182. Following startup of the equipment and a few rotations of belt 170, the coating liquid wets the surfaces of pick-and-place rolls 172, 174, 176, 178, 180 and 182. The liquid coating splits at the trailing end (the lift-off points) of the liquid-filled nip regions where belt 170 contacts pick-and-place rolls 172, 174, 176, 178, 180 and 182. A portion of the coating remains on the pick-and-place rolls 172, 174, 176, 178, 180 and 182 as they rotate away from the lift-off points. The remainder of the coating travels onward with belt 170. Variations in the coating caliper just prior to the lift-off points will be mirrored in both the liquid caliper variation on belt 170 and on the surfaces of the pick-and-place rolls 172, 174, 176, 178, 180 and 182 after they leave lift-off points. Following further movement of belt 170, the liquid on the pick-and-place rolls 172, 174, 176, 178, 180 and 182 will be redeposited on belt 170 in new positions along belt 170.
The embodiment of
When the amount of liquid necessary for the desired average coating caliper is applied intermittently to wet belt 170 or to some other target substrate, the period and number of pick-and-place rolls preferably is chosen to accommodate the largest spacing between any two adjacent, down web deposits of coating. A significant advantage of such a method is that it is often easy to produce heavy cross web stripes or zones of coating on a belt or other target substrate but difficult to produce thin, uniform and continuous coatings. Another important attribute of such a method is that it has pre-metering characteristics, in that coating caliper can be controlled by adjusting the amount of liquid applied to the belt or other target substrate.
Although a speed differential can be employed between belt 170 and any of the other rolls shown in
As mentioned in connection with
Coating liquids can be applied in a variety of uneven patterns other than stripes, and by using methods that involve or do not involve contact between the applicator and the surface to which the coating is applied. For example, an oscillating needle applicator such as described above in connection with
If a fixed flow rate to a drop-producing device is maintained, the substrate translational speed is constant, and most of the drops deposit upon the substrate, then the average deposition of liquid will be nearly uniform. However since the liquid usually deposits itself in imperfectly spaced drops, there will be local variations in the coating caliper. If the drop deposition frequency is low or the drop size is low, the drops may not touch, thus leaving uncoated areas in between. Sometimes these sparsely placed drops will spontaneously spread and merge into a continuous coating, but this may take a long time or occur in a manner that produces a non-uniform coating. The use of exactly uniform or substantially uniform contact roll periods is especially useful for improving sparsely deposited droplet- or spray-deposited thin coatings. If the drops in such coatings do not overlap, the total length of all the wetting contact lines around all the individual drops will be very large. The act of contacting the drop-covered substrate surface with a roll is immensely powerful in speeding drop spreading. The resulting enhancement in the rate of drop spreading and wetting will be independent of the rotational period of the rolls and will primarily be influenced by the total wetting line length present. In contrast to coatings applied using a stripe coater, the wetting line length per unit area will be orders of magnitude greater for a coating applied as sparsely deposited drops. For example, if droplets are deposited on a one meter wide web in square, sparse arrays with one millimeter spacing and coverage of 10 percent of the web surface, then the drops in total will have a perimeter length (a cumulative wetting line length) of 1,120 meters per square meter of web surface. As the percent coverage approaches 100%, the wetting line length approaches 4 million meters per square meter of web surface. If a single stripe is applied at 10 percent coverage parallel to two of the edges of a 1 meter square piece of web, the total wetting line length will be 2 meters. As the stripe coverage approaches 100%, the wetting line length will remain at 2 meters. Thus the use of a roll to bring about an enhanced spreading rate can be vastly more important for drops than for stripes. Enhancement of spreading by translation of the wetting line amounts to a second mechanism of uniformity improvement in addition to the pick and place liquid separation/replacement mechanism already described above. This wetting line spreading mechanism is not primarily dependent upon the roll size or size uniformity. Instead, it primarily depends on the presence of contacting devices. If the spraying deposition rate is large enough to produce a continuous coating, the statistical nature of spraying will produce non-uniformities in the coating caliper. Here too, the use of rolls or other selected periodic pick-and-place devices can improve coating uniformity.
Accordingly, an improvement station of the present invention can be advantageously used with a non-uniform coating, e.g., a coating of stripes or drops. The improvement station can convert the non-uniform coating to a continuous coating, or improve the uniformity of the coating, or shorten the time and machine length needed to accomplish spreading, and especially drop spreading. The act of contacting discontinuous drops with rolls or other selected periodic pick-and-place devices, removing a portion of the drop liquid, then placing that removed portion back onto the substrate in some other position increases the surface coverage on the substrate, reduces the distance between coated spots and increases the drop population density. The contacting action also creates pressure forces on the drop and substrate, thereby accelerating the rate of drop spreading. Contact in the area around and at a drop may produce a high liquid interface curvature at or near the spreading line and thereby enhance the rate of drop spreading. Thus the use of selected periodic pick-and-place devices makes possible rapid spreading of drops applied to a substrate and improves the uniformity of the final coating.
Spraying can be accomplished using many different types of devices. Examples include point source nozzles such as airless, electrostatic, spinning disk and pneumatic spray nozzles. Line source atomization devices are also known and useful. The droplet size may range from very large (e.g., greater than 1 millimeter) to very small. The nozzle or nozzles can be oscillated back and forth across the substrate, e.g. in a manner similar to the above-described needle applicator. Particularly preferred drop deposition devices are described in copending U.S. patent application Ser. No. 09/841,380 entitled ELECTROSTATIC SPRAY COATING APPARATUS AND METHOD and Ser. No. 09/841,381 entitled VARIABLE ELECTROSTATIC SPRAY COATING APPARATUS AND METHOD (now U.S. Pat. No. 6,579,574 B1), both filed Apr. 24, 2001, the entire disclosures of which are incorporated by reference herein.
The beneficial application of the periodic pick-and-place devices of the present invention can be tested experimentally or simulated for each particular application. Many criteria can be applied to measure coating uniformity improvement. Examples include caliper standard deviation, ratio of minimum (or maximum) caliper divided by average caliper, range (defined as the maximum caliper minus the minimum caliper over time at a fixed observation point), and reduction in void area. For example, through the use of the present invention, range reductions of greater than 75%, greater than 80%, greater than 85% or even greater than 90% can be obtained. For discontinuous coatings (or in other words, coatings that initially have voids), the invention enables reductions in the total void area of greater than 50%, greater than 75%, greater than 90% or even greater than 99%. The application of this method can produce void-free coatings. Those skilled in the art will recognize that the desired degree of coating uniformity improvement will depend on many factors including the type of coating, coating equipment and coating conditions, and the intended use for the coated substrate.
Through the use of the invention, 100% solids coating compositions can be converted to void-free or substantially void-free cured coatings with very low average calipers. For example, coatings having thicknesses less than 5 micrometers, less than 1 micrometer, less than 0.5 micrometer or even less than 0.1 micrometer can readily be obtained. Coatings having thicknesses greater than 5 micrometers can also be obtained. In such cases it may be useful to groove, knurl, etch or otherwise texture the surfaces of one or more (or even all) of the pick-and-place devices so that they can accommodate the increased wet coating thickness.
As discussed above, one aspect of the invention involves first applying stripes interspersed with voids and then using rolls to pick and place the applied liquid and create a continuous coating. These stripes may extend from one edge to the other edge of a continuous web, or they may extend only across one or more of a number of down web lanes. Further understanding of this aspect of the invention and the manner in which stripe periods and roll diameters can be selected can be obtained by reviewing
Every point on the improvement diagram in
b presents the information of
Using
a and
c is an improvement diagram in the form of a linear continuous gray scale plot that identifies preferred and more preferred roll sizes as a function of stripe width for a system using a single roll. As with the improvement diagram shown in
e is an improvement diagram in the form of a linear continuous gray scale plot that identifies preferred and more preferred roll sizes as a function of stripe width for a system using two rolls. As with the improvement diagrams shown in
g is an improvement diagram in the form of a linear continuous gray scale plot that identifies preferred and more preferred roll sizes as a function of stripe width for a system using three rolls. As with the improvement diagrams shown in
i is an improvement diagram in the form of a linear continuous gray scale plot that identifies preferred and more preferred roll sizes as a function of stripe width for a system using four rolls. As with the improvement diagrams shown in
k is an improvement diagram in the form of a linear continuous gray scale plot that identifies preferred and more preferred roll sizes as a function of stripe width for a system using five rolls. As with the improvement diagrams shown in
m is an improvement diagram in the form of a linear continuous gray scale plot that identifies preferred and more preferred roll sizes as a function of stripe width for a system using ten rolls. As with the improvement diagrams shown in
The discussions above have focused mainly on cases in which all the pick-and-place device periods were exactly equal with a precision of one part in approximately 10,000. Simulation experiments show that lessening this precision will influence the predicted results, generally in a favorable manner. It can be advantageous at times to employ nominally identically rolls that have measurable variations in their rotational periods. This may be accomplished in many ways.
In the laboratory or factory all mechanical parts have some limit of precision. All rotating machinery has some limit to the accuracy of the rotational instantaneous speed and the periods of successive revolutions. The resulting deviations from the nominal or set values may have very profound influences on actual experimental results or model simulations. When rolls are manufactured their cost is directly related to the precision of manufacture. Inexpensive metal and plastic rolls on the order of 25 millimeters in diameter may have a precision as poor as plus or minus 0.1 millimeters. Rubber rolls may have a precision as poor as plus or minus 0.5 millimeters. The wear and abuse of these rolls with continuing use can often further degrade their precision. This imprecision is actually beneficial for improving coating uniformity via a train of pick-and-place devices.
For driven rolls, the rotational period of a roll is influenced by its diameter and the mechanism used to drive the roll. The movement of a web past an undriven roll may turn the roll, negating the need for a drive motor. This is the least expensive and simplest mechanical configuration. In such cases factors such as the web speed, friction or traction forces between the web and the roll, and forces retarding rotation such as bearing friction or brake drag govern the rotational rate. When the angle of wrap of the web on a roll is low, there can be increased frictional slippage between the roll and web (or increased traction slippage if a liquid fills the contact area). If the rotational driving forces are nearly balanced by the retarding frictional forces then changes in the frictional forces will measurably influence the rotation speed of the roll. Variations may occur in the measured rotational period or in the instantaneous rate of rotation.
Typically, efforts to improve caliper uniformity with other coating methods have required very precise bearings and very careful control of line speeds, roll diameters and other variables. In contrast, the present invention demonstrates that some degree of imprecision in the diameters of pick-and-place rolls can be useful. Expressed more generally, imprecision in the rotational period of a set of pick-and-place devices, for whatever reason, may be useful. These variations have utility for improving coating uniformity. Even very small variations in the relative speeds or periodicity of a set of pick-and-place devices, or between one or more such devices and a substrate, are useful for enhancing performance. Random or controlled variations can be employed. For example, in a train of at least 3 rolls having nominally uniform periods, it can be desirable for at least 2 rolls to have actual variations in their periods between about 2% and about 10%. Likewise, in a train of at least 5 rolls having nominally uniform periods, it can be desirable for at least 2 rolls to have actual variations in their periods between about 0.1% and about 10%. Variation of the periods can be accomplished, for example, by independently driving the rolls or other devices using separate motors and varying the motor speeds. Those skilled in the art will appreciate that the speeds of rotation can also be varied in other ways, e.g., by using variable speed transmissions, belt and pulley or gear chain and sprocket systems where a pulley or sprocket diameter is changed, limited slip clutches, brakes, or rolls that are not directly driven but are instead frictionally driven by contact with another roll. Periodic and non-periodic variations can be employed. Non-periodic variations can include intermittent variations and variations based on linear ramp functions in time, random walks and other non-periodic functions. All such variations appear to be capable of improving the performance of an improvement station containing a fixed number of rolls. Improved results are obtained with variations as low as 0.2 percent of the average, and more preferably at least 0.4 percent of the average.
The advantages of such small variations can be better illustrated with the following example. In gravure coating inadequate flooding of the gravure roll prior to doctoring, or the entrainment of air bubbles in the coating liquid, can cause random voids in the coating. With a 300 mm diameter gravure roll, voids of 1 millimeter can be readily and inadvertently generated. The voids of this example are not periodically reoccurring. An improvement station containing a series of rubber-covered pick-and-place rolls having a nominal 200 mm circumference can dramatically reduce the defects caused by such voids.
Extensive modeling has yielded additional insights into the problem of healing random defects. Improvement in coating uniformity is governed in part by a ratio calculated by determining the absolute value of the maximum variation in the roll period from the average roll period, and dividing by the defect size.
Small variations in the periods of pick-and-place devices can also heal repeating periodic defects. Such defects are often generated by operational problems with roll coating devices. For example, in gravure coating one or more cells of the patterned roll can become plugged. This can be caused by drying of a coating formulation on a portion of the gravure roll or filling of one or more of the cells with particulates. In either case, the plugged cell or cells can continuously produce a defective low coating weight spot on the web for each rotation of the gravure roll. In the worst case this results in periodic voids extending down web for the continued duration of the coating process.
The period of a pick-and-place roll can be varied in a variety of ways besides initial imprecision in the roll diameter. For example, roll diameter can be statically changed (e.g., by replacing a roll, with or without interruption of a coating operation) or dynamically changed (e.g., by inflating or deflating or otherwise expanding or shrinking the roll while maintaining the roll's surface speed and without interrupting a coating operation). The rolls do not have to have constant diameters; if desired they can have crowned, dished, conical or other sectional shapes. These other shapes can help adjust the periods of a set of rolls. Also, the position of the rolls or the substrate path length between rolls can be varied during operation. One or more of the rolls can be positioned so that its axis of rotation is not perpendicular (or is not always perpendicular) to the substrate path. Such positioning can improve performance, because such a roll will tend to pick up coating and reapply it at a laterally displaced position on the substrate. All of the above variations are useful, and all can be used to affect and improve the performance of the improvement station and the uniformity of the caliper of the finished coating. For example, if partial plugging of a gravure roll pattern occurs during a coating run, then the resulting defects can be overcome without halting the run by using one of the above described variation techniques to impart an appropriate compensatory variation in rotational speed of one or more of the improvement rolls relative to the web.
In addition to varying the period of one or more pick-and-place devices as described above, coating uniformity can also be improved by varying the input period or size of a repeating defect. For example, the rotational speed of a gravure roll coater or other roll coating device can be changed to alter the input frequency of periodic defects associated with the roll coating device. Likewise, the period of a stripe coater can be changed to alter the stripe frequency or the interval between coating stripes. By monitoring the uniformity of the coating exiting the improvement station and making appropriate adjustments in the input defect period or size, overall coating uniformity can be significantly improved.
a through
In
If one knows or can measure the most probable size of a repeating defect, then it is possible to choose a set of rolls with deliberately chosen period deviations (size deviations) that provide a dimensionless void size to roll period deviation ratio less than one. Such a roll set will provide improved uniformity compared to a roll set in which the dimensionless void size to roll period deviation ratio is greater than one. Improved uniformity can also be attained by using other measures to reduce the dimensionless void size to roll period deviation ratio to a value less than one. For example, one can use rolls nominally of the same size but having larger dimensional tolerances. Another measure would be to vary slightly the rotational speeds of the rolls. If the rolls are not driven, then as mentioned above their traction with the web may be altered or frictional braking may be applied. If the rolls are constructed from thermally expanding materials, then the roll sizes (and the roll period deviation) can be modified by operating the rolls at differing temperatures.
Detailed simulation investigations have also revealed that the performance of the improvement rolls of the invention can be altered in unexpected ways. For example, FIG. 20 through
In each of FIG. 20 through
For coatings containing random rather than repeating voids and an improvement station employing 5 or more substantially uniform rolls, the improvement in uniformity is generally better if the substantially uniform rolls vary in size by an amount greater than 0.5 times the void size. For such random voids the average roll size will be unimportant. Instead, the number of rolls, the random void size and the roll period variations primarily influence the uniformity results. For example, as shown above in connection with
A coating having random or periodic areas that are deficient in coating material can be analyzed by considering the coating to be made up of a uniform base coating underneath a voided coating of the same composition. The improvement devices described herein will act to remove and reposition the top voided coating in a manner similar to their action on a lone voided coating. Thus the teachings provided herein for a voided coating also apply to a non-voided but non-uniform coating containing coating depressions. In a similar manner periodic or random excesses in a coating can be analyzed by considering the coating to be made up of a uniform base coating underlying a discontinuous top coating. Thus the teachings provided herein for a voided coating also apply to a non-voided but non-uniform coating containing coating surges.
As mentioned above, another aspect of the invention is that the improvement station increases the rate of drying volatile liquids on a substrate. Drying is often carried out after a substrate has been treated by washing or by passage through a treating liquid. Here the main process objective is not to apply a liquid coating, but instead to remove liquid. For example, droplets, patches or films of liquid are commonly encountered in web processing operations such as plating, coating, etching, chemical treatment, printing and slitting, as well as in the washing and cleaning of webs for use in the electronics industry.
When a liquid is placed on or is present on a substrate in the form of droplets, patches, or coatings of varying uniformity and if a dry substrate is desired, than the liquid must be removed. This removal can take place, for example, by evaporation or by converting the liquid into a solid residue or film. In industrial settings drying usually is accomplished using an oven. The time required to produce a dry web is constrained by the time required to dry the thickest caliper present. Conventional forced air ovens produce uniform heat transfer and do not provide a higher drying rate at locations of thicker caliper. Accordingly, the oven design and size must account for the highest anticipated drying load.
The improvement stations of the invention substantially reduce the time required to produce a dry substrate, and substantially ameliorate the effect of coating caliper surges. The improvement station diminishes coating caliper surges for the reasons already explained above. Even if the coating entering the improvement station is already uniform, the improvement station greatly increases the rate of drying. Without intending to be bound by theory, the repeated contact of the wet coating with the pick-and-place devices is believed to increase the exposed liquid surface area, thereby increasing the rate of heat and mass transfer. The repeated splitting, removal and re-deposition of liquid on the substrate may also enhance the rate of drying, by increasing temperature and concentration gradients and the heat and mass transfer rate. In addition, the proximity and motion of the pick-and-place device to the wet substrate may help break up rate limiting boundary layers near the liquid surface of the wet coating. All of these factors appear to aid in drying. In processes involving a moving web, this enables use of smaller or shorter drying stations (e.g., drying ovens or blowers) down web from the coating station. If desired, the improvement station can extend into the drying station.
The methods and devices of the invention can be used to apply, make more uniform or dry coatings on a variety of flexible or rigid substrates, including paper, plastics, glass, metals and composite materials. The substrates can be substantially continuous (e.g., webs) or of finite length (e.g., sheets). The substrates can have a variety of surface topographies including smooth, textured, patterned, microstructured and porous surfaces (e.g., smooth films, corrugated films, prismatic optical films, electronic circuits and nonwoven webs). The substrates can have a variety of uses, including tapes, membranes (e.g., fuel cell membranes), insulation, optical films or components, electronic films, components or precursors thereof, and the like. The substrates can have one layer or many layers under the coating layer. The invention is especially useful for converting a discontinuous coating (such as one applied using above-described stripe coater) into a continuous coating.
The invention is further illustrated in the following example, in which all parts and percentages are by weight unless otherwise indicated.
Using a modified coating machine equipped with an improvement station of the invention, a plastic web was coated with intermittent, periodic and sparsely applied cross web stripes of a coating liquid, then converted to a web having a continuous uniform coating. The web was 0.05 mm thick and 51 mm wide biaxially oriented polyester film. The coating liquid contained 2600 parts by volume of glycerin, 260 parts by volume of isopropyl alcohol, and 1 part by volume of a fluorochemical wetting agent (3M™ FLUORAD™ FC-129 fluorosurfactant, Minnesota Mining and Manufacturing Company, St. Paul, Minn.). The coating liquid was applied to a transfer roll and then transferred to the web. The coating station employed an air driven oscillating mechanism that stroked a flexible polypropylene needle back and forth across the transfer roll. The oscillating mechanism was a Model BC406SK13.00 TOLOMATIC™ Pneumatic Band Cylinder with a linear actuator (Tol-O-Matic, Inc., Hamel, Minn.). The coating liquid was premetered using a syringe pump obtained as model 55-1144 from Harvard Apparatus. The polypropylene needle had a 0.48 mm tip and was obtained as part number 560105 from I & J Fisnar Inc. Interconnection between the syringe pump and the needle was made using flexible, 4 mm OD plastic tubing. The needle was positioned so that the needle tip contacted with the transfer roll.
The transfer roll was 62.7 mm in diameter and was driven by contact with and movement of the web. Using a web speed of 7.77 meters per minute, a liquid flow rate of 0.5 ml/min., a stroke rate of 120 per minute and a stroke length of 127 mm, a pattern of narrow, crosshatched stripes was premetered onto the web at a rate sufficient to provide an overall average coating caliper of 0.5 micrometers.
The coated web was then brought into contact with an improvement station containing 25 undriven corotating rolls. The improvement station rolls were obtained from Webex Inc. as dynamically balanced aluminum dead shaft rolls with smooth anodized roll faces, a face length of 355.6 mm, and nominal diameters of 50.8 mm. Actual measurements of the roll diameters showed that 1 roll had a diameter of 49.42 mm, 3 rolls had a diameter of 49.40 mm, 2 rolls had a diameter of 49.36 mm, 13 rolls had a diameter of 49.34 mm, 1 roll had a diameter of 49.33 mm and 5 rolls had a diameter of 49.28 mm. The resulting set thus had an average diameter of 49.36 mm, with 5 rolls in the set having a diameter that was 0.2% less than the average diameter and 1 roll in the set having a diameter that was 0.1% more than the average diameter. Each roll was wrapped by the web for at least 30 degrees of the roll circumference. Using a hand held mechanical tachometer, no variation in roll versus web speed could be found.
Following passage through the improvement station, the very discontinuous initially applied coating was transformed to a continuous, void-free but patterned coating. As observed using the unaided naked eye, the pattern exhibited crosshatched overlapping areas of heavy coating with areas of lighter coating in between. Evaluated visually, the overall variation appeared to be approximately ±50% of the average caliper. In order to obtain a more uniform coating, the web was next passed around a 76.2 mm diameter air turn bar positioned so that its axis was coplanar with but angled to the axis of the preceding improvement roll. One 360° revolution around the air turn bar produced a sideways offset for the web path greater than the width of the web. By using several transitional idler rolls to turn the web back in the direction of the improvement station, the coated web could be brought back into contact with the improvement station rolls on a path parallel to but not overlapping the original web path. The net result was to enable the coated side of the web to make contact and re-contact 50 times with nearly identical rolls. Following this second pass through the improvement rolls, the coated web appearance was visibly void-free, pattern free, and uniform. Accordingly, the improvement station provided a significant increase in coating uniformity.
Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. This invention should not be restricted to that which has been set forth herein only for illustrative purposes.
This application is a continuation-in-part of application Ser. No. 09/757,955 filed Jan. 10, 2001, entitled COATING DEVICE AND METHOD (now U.S. Pat. No. 6,737,113 B1), the entire disclosure of which is incorporated by reference herein.
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| Number | Date | Country | |
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 09757955 | Jan 2001 | US |
| Child | 10044237 | US |
