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Ind. Eng. Chem. Fundam. lQ81, 20, 63-66
Reverse Roll Coating of Viscous and Viscoelastic Liquids Jehuda Greener Eastman Kodak Company, Rochester, New York 14650
Stanley Middleman’ Department of Applied Mechanics and Engineering Sciences, Unlversity of California, San Dlego, La Jolla, California 92093
Reverse-roll coating is described, and a simple lubrication-type model is developed with which one may predict relationships among several geometrical and dynamic parameters. Observations of film thickness over a range of speed ratios from 0.25 to 1.75 are In agreement with the trends predicted by the model. Elastic polymer solutions do not exhibit unusual coating performance characteristics. No ribbing instability is observed in reverse-roll coating, even with strongly elastic liquids.
Roll coating systems are common in a host of industrial operations and are used for continuous deposition of thin layers of fluids onto moving surfaces. In general, rolling elements allow easy application of the coating fluid onto the substrate with a good degree of control over the thickness of the coating layer (Booth, 1968). A common problem, however, with roll coating devices is the onset of surface instabilities on the coating layer which are undesirable and should be eliminated in order to obtain a smooth surface finish. (See, for example, Matsuda and Brendly, 1979). An illustration of the kind of surface irregularities which may arise in a common roll coating operation is shown in Figure 1. Above some critical speed the surface starts to rib and deform; the ribs run in an orderly fashion in the direction of motion and give the surface a wavy appearance. This phenomenon, which is referred to as the ribbing instability, has long been recognized for the case of Newtonian coatings (Mill and South, 1967; Pearson, 1960; Pitts and Greiller, 1961). Our own studies of this problem (Greener, 1978; Greener et al., 1980) have shown that viscoelastic fluids are far less stable than Newtonian fluids with fluid elasticity playing a major role in the onset of the ribbing instability. For some fluids, and particularly viscoelastic ones, the ribbing is so pervasive that it is simply not feasible to eliminate the ribs by common means such as lowering the speed; in many cases the critical speeds are too low to make the operation economical (Mill and South, 1967). One approach that is commonly used in industry to overcome this problem is to reverse the direction of rotation of the roll relative to the web; Le., have the roll and the web surfaces move in opposite directions. This version of the roll-coating system is called reverse-roll coating (RRC). Reverse roll coating is used both in metering or finish applications and in pre-metering or tranfer applications, and it yields coatings which are usually smooth and rib free. Figure 2 shows a sketch of an RRC system. The system is characterized by several parameters: the main geometric parameters are the nip separation (Wo) and the radius (or radii) of the roll (or rolls). The speed ratio ( K ) and the thickness of the coating on the reverse roll (H,) are typical operational parameters, and the coating thickness on the substrate or forward roll (Hf)is a typical performance variable. The flow problem associated with this process is not trivial due to the two-dimensional character of the flow in the metering area. We were able, 0196-4313/81/1020-0063$01 .OO/O
in fact, to detect (visually) strong circulatory motion in the metering zone with a clearly visible stagnation point. Figure 3 is a schematic representaton of a streamline pattern in the RRC system based upon our visual observations, and it clearly shows that far from the nip, in the metering area, the flow is two-dimensional. A Lubrication Model In the analysis of this problem it is assumed that the lubrication approximations hold in the nip region; i.e., the flow is nearly parallel so that
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