Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2001-03-05
2003-02-25
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S420000, C427S428010
Reexamination Certificate
active
06524660
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to apparatus and methods for coating a liquid composition onto a moving substrate to form a coated layer thereon; and, more particularly, to coating apparatus and methods utilizing a backing roller while providing an electrostatic field at the dynamic wetting line where the coating liquid meets the moving substrate.
BACKGROUND OF THE INVENTION
In coating a liquid composition from a coating die, hopper, or similar coating device onto a first or “front” surface of a moving web substrate, it is well known in the coating art to precisely position and support the substrate by guiding the substrate around a rotating backing roller spaced apart from the coating device. The distance between the front surface of the web and the coating device is referred to as the “coating gap.” The web is thus supported directly by the surface of the backing roller through a substantial angle of rotation, or “wrap,” typically between about 90° and 180°.
The front and back surfaces of the moving web carry boundary layers of air, each of which can create different problems in achieving stable coatings at high coating speeds. In the prior art, the preferred solutions to these differing front and back surface problems can be mutually incompatible.
The boundary air layer on the back surface of the web is drawn into the entrance nip formed between the web and the backing roller, which in the older prior art is typically a smooth-surface roller. At lower conveyance speeds, for example, 0.5 m/s, the air is squeezed out at the nip by tension in the web, and the web is supported without slippage on the roller. However, as conveyance speed is increased, the boundary air is incompletely squeezed out and the web begins to float on a dynamic cushion of air between the web and the roller and thus traction between the web and roller diminishes. This can lead to at least three unwanted effects: the web may wander laterally on the roller, resulting in intermittent honing of the web back surface and coating off the edge of the web; the web may not turn synchronously with the backing roller, resulting in scratching or honing of the web back surface and irregularly variable web speed at the coating point; and the coating gap may be decreased irregularly and unacceptably by the air cushion, causing unpredictable and unacceptable thickness variations in the coating.
It is well known in the prior art to relieve the back side boundary air layer by providing any of various incuse patterns in the surface of the backing roller. These patterns may include, for example, a random surface comprising lands and incuse areas which may be varied in the percentage of surface area occupied by each (see for example U.S. Pat. No. 4,426,757 to Hourticolon, et al.). More commonly, an axially central portion the roller is circumferentially scribed with a pattern of shallow grooves. See, for example, U.S. Pat. No. 3,405,855 to Daly et al. and U.S. Pat. No. 4,428,724 to Levy. Such circumferential grooves are known in the photographic coating art as “microgrooves” and may take the form either of a plurality of truly circumferential closed grooves, each in a plane orthogonal to the roller axis, or of a single continuous spiral groove of appropriately shallow pitch. The performance of these two groove patterns is substantially equivalent. A pattern commonly in use in the coating of photographic products employs 1 groove per axial millimeter (gpmm) of roller surface, each groove being 0.3-0.6 mm wide at the roller surface and about 50 to 130 &mgr;m deep (see U.S. Pat. No. 6,177,141 to Billow, et al.). This pattern, provided over an axially central portion of a backing roller, can provide suitable traction and conveyance stability of a flexible plastic web substrate around a coating backing roller about 10 to 20 cm in diameter at linear speeds exceeding 5 m/s, unit area traction being substantially increased over that exhibited by a smooth roller despite the loss in roller surface area available for contact with the web.
The front surface boundary air layer can create a similar problem in engaging the coating composition as it is being applied from a hopper to the web surface. As coating speed is increased, a critical speed is encountered at which air begins to be entrained under the coating composition at the coating point, preventing the composition from wetting the web along a uniform line and thus unacceptably disrupting the uniformity of coating. It is well known in the coating art that imposing an electric field between the front surface of the web substrate and the hopper can raise significantly this critical speed for air entrainment (AE), for example, from about 2 m/s to about 6 m/s (see for example U.S. Pat. No. 4,837,045 to Nakajima). This technique is referred to as electrostatic assist for coating (ESA).
A serious problem can arise, however, in using ESA when coating onto a web supported by a grooved backing roller. A periodic coating thickness non-uniformity, referred to herein as groove lines, tends to form in the lower liquid layers as they are applied to the web, the lines being an image of the backing roller surface pattern. The electrostatic force generated on the coating composition is proportional to the square of the imposed electric field (E
2
). Therefore, it follows that the magnitude of coating nonuniformity is proportional to any variation in E
2
occurring in the immediate vicinity of the lower surface of the coating composition as it is contacting the web. The electric field is inversely proportional to the dielectric gap between the roller surface and the front surface of the web understanding that over land areas of the roller, the gap is simply the thickness of the web, whereas in grooved areas, the gap includes the depth of the grooves. Thus there exists a pattern of periodic variation in electric field, and ESA, exerted on the coated fluid along the axial direction of the roller, creating a groove line pattern in the coating.
Multi-layer coating packs or composites having a relatively low bottom layer viscosity, for example, 4 centipoises (cP), are especially prone to formation of groove lines. As coating speed or viscosity is increased, the prevention of front-side air entrainment, even with a grooved coating backing roller, typically requires progressively higher voltages of ESA, which can, in turn, result in more intense groove lines in the coating. Thus, in the known art, grooving the backing roller to relieve the back side boundary layer problem is antithetical to increasing ESA voltage to relieve the front side boundary layer problem. The propensity to form groove lines is thus a serious impediment to achieving high coating speeds (in excess of 2 m/s) or high viscosity (in excess of 10 cP at 10
5
reciprocal seconds) as may be desirable for increased productivity and coated uniformity.
Another approach to relieving the back side boundary layer problem is to use a nip roller to press the web against a smooth coating backing roller and squeeze out the air entrained between the web and the roller. This nip roller would be located prior to the coating application point. The use of a smooth backing roller would avoid creation of non-uniform ESA. However, this nip roller would need to contact the face side of the web immediately prior to coating. In many situations, it is desirable to avoid contact with the face side of the web until the last layer of coating has been applied and sufficiently dried. In addition, the use of a nip roller increases the chances of causing creasing, particularly with thinner webs.
In prior art practice, using a prior art backing roller having a pitch of 1 gpmm and a groove depth of 130 &mgr;m and a groove width of 500 &mgr;m, for a given web substrate having a given thickness and being coated at a given web speed, the level of ESA is adjusted until a very low but acceptable intensity of groove lines is achieved. Typically, the coating speed and the ESA level are co-optimized to achieve the maximum possible coating speed with the high
Lioy Daniel C.
Quiel Robert R.
Ramos, Jr. Fernando
Wakefield David A.
Zaretsky Mark C.
Bocchetti Mark G.
Eastman Kodak Company
Parker Fred J.
LandOfFree
System for coating using a grooved backing roller and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System for coating using a grooved backing roller and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System for coating using a grooved backing roller and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3163728