Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1999-09-29
2001-01-09
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S420000, C427S532000
Reexamination Certificate
active
06171658
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods for coating liquids onto moving substrates, more particularly to methods for increasing the coating speed and uniformity of mixtures or solutions using electrostatic assistance.
BACKGROUND OF THE INVENTION
This invention relates in general to the art of coating and in particular to an improved method for carrying out a process of coating in which one or more layers of coating composition, preferably a conductive composition, are applied to the surface of a substrate by advancing the substrate through a coating zone in which a flow coating composition is applied thereto, for example, a process of bead coating or a process of curtain coating. More specifically, this invention relates to an improved coating method in the manufacturing of a photographic film, photographic paper, photographic printing layer, a magnetic recording tape, an adhesive tape, pressure-sensitive recording layer, an offset printing plate material or the like.
A method of applying an electrostatic force to assist in a coating method, along with a conventional method of coating a continuously moving web, has been previously disclosed. For example, as disclosed by Hartman in WO 89/05477, ionizers may be used to deposit polar charge on the web prior to the coating application locus to generate an electrostatic field at the coating application locus for a curtain coating method. This electrostatic assist enables the coating method to operate at increased speeds without the defect of air bubbles trapped in the coating layers or between the web and the coated layer. Many prior patents are cited by Hartman discussing the use of polar charge assist in a bead coating method, as well as methods of measuring and controlling the electrostatic field so that a uniform charge of the required magnitude is obtained. These patents do not describe any particular electrical properties of the web that are particularly helpful to the use of electrostatic assist for a coating method.
In another example disclosed by De Geest in U.S. Pat. No. 3,335,026, a potential difference is applied between the coating roller and the coating composition to generate an electrostatic field to attract the coating composition to the web. This patent constrains the resistivity of the web surface to be greater than 10
9
ohms/square. However, as is shown below in the description of the present invention, it is not the surface resistivity alone, but its combination with the web speed and web capacitance while on the coating roller that determines the effectiveness of the electrostatic assist. Thus, it is possible to use electrostatic assist for web surfaces having a resistivity less than 10
9
ohms/square. Furthermore, De Geest does not address the issue of designing a support with respect to surface resistivity and web capacitance so as to achieve a specified coating speed using electrostatic assist with minimized coating roller voltage levels. By minimized coating roller voltage levels it is meant that the voltage level is preferably as close as possible to the voltage level required when using an insulating web having a surface resistivity greater than 10
13
ohms/square.
In another example disclosed by Nakajima in U.S. Pat. No. 4,837,045, an electrostatic force on a coating composition is combined with a web having a gelatin-subbing layer containing a surfactant. This electrostatic force allows an increase in speed of coating without increasing the load of drying the coated layers. The gelatin-subbing layer is required to contain a surfactant to achieve the desired electrostatic assist.
In another example disclosed by Kawanishi in U.S. Pat. No. 4,835,004, web temperature control is used to reduce the non-uniformity of and preserve the level of electrical charges deposited on the web by a set of ionizers prior to the coating application locus. This uniform charge is then used to provide an electrostatic assist for the coating method to yield defect-free coatings. This patent places certain requirements on the environment (temperature, relative humidity (RH)) of the web prior to the coating application locus to achieve a uniform charge.
Thus, there is a need for a method for coating emulsions at high speeds without having air bubbles entrained in the coated layer and with no loss in adherence using electrostatic assist regardless of the presence, or absence, of surfactants in a gelatin subbing layer and without placing restrictions on the environment of the web prior to the coating application locus.
SUMMARY OF THE INVENTION
Accordingly, several objects and advantages of the present invention are:
1) to provide a coating method that utilizes an electrostatic force to increase coating speed without modifying the coating composition and without suffering from entrained air bubbles in the coated layers or between the web and the coated layers;
2) to provide a coating method that ensures the existence of an electrostatic force at the coating application locus without the need for surfactant-containing gelatin subbing layers;
3) to provide a methodology for designing a support with respect to surface resistivity and web capacitance so as to achieve a specified coating speed using electrostatic assist with minimized coating roller voltage levels.;
4) to minimize the voltage or charge level required at the coating point for a desired electrostatic force to minimize the possibility of arcing and glow during or after coating and to minimize electrostatic charge remaining on the web after coating.
The above and other objects of the present invention can be achieved through use of a fundamental parameter, the characteristic electrical length, &lgr;, expressed in micrometers (&mgr;m), determined by the electrical properties of the web and the coating speed of the web. The relationship of &lgr; to a voltage applied to the coating roller, as shown in
FIGS. 2 and 3
, permits for the first time the calculated placement of an intended coating at an optimally robust level of electrostatic assist.
The characteristic electrical length is defined as the reciprocal of the product of the web surface resistance (ohms/square), web capacitance while on the coating roller (F/m
2
)—defined as the ratio of the web permittivity (F/m) divided by the web thickness (m), and the web speed (m/s). A characteristic electrical length less than 400 &mgr;m, and preferably less than 100 &mgr;m, is desirable. When meeting this criterion, the web surface voltage in the vicinity of the coating point (within 100 &mgr;m) remains at a level sufficient to apply a significant electrostatic force on the coating composition. This criterion is independent of whether the electrostatic force is applied via polar charge deposited on the web or a potential difference applied between the coating roller and the coating composition or a combination of these two methods.
The failure mode of entrained air in the coating is encountered at some point as coating speed is increased. This failure mode can be suppressed until higher speeds by the application of an electrostatic force between the fluid and web. Achieving this force requires an electrostatic charge or electrostatic voltage source as well as some constraints on the electromechanical properties of the web, both bulk and surface-to-be-coated. The present invention properly provides these constraints, ensuring the full effectiveness of the electrostatic charge or voltage. Coatings made in accordance with the invention are not dependent upon the use of any particular surfactant in the gelatin layer on the surface of a web to be coated, nor are they dependent upon control of the environment (RH, temperature) the web encounters prior to the coating process.
REFERENCES:
patent: 3335026 (1967-08-01), De Geest et al.
patent: 3730753 (1973-05-01), Kerr
patent: 4835004 (1989-05-01), Kawanishi
patent: 4837045 (1989-06-01), Nakajima
patent: 89/05477 (1989-06-01), None
Billow Steven A.
Whitney Roger A.
Zaretsky Mark C.
Bocchetti Mark G.
Eastman Kodak Company
Parker Fred J.
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