Coating reverse-roll

Different types of cells on the surface of a gravure roll (a) pyramidal, (b) quadrangular, (c) trihelical. (Courtesy of Flexographic Technical Association.) 

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Two methods of plastisol coating are shown in Figure 2 depending on whether a flat surface (using a knife-on-roller method) or even-thickness coating (reverse roll coating) is required. 

Roll coating is widely used as an efficient way to coat uniform, generally flat or cylindrical surfaces of rigid or flexible substrates. There are many types of roll coating methods the two most common are direct roll coating and reverse roll coating. 

Coyle, D.J., C,W, Macosko, and L.E. Scriven Fluid Dynamics of Reverse Roll Coating Amer. Inst, of Chem. Eng s J., 16 (February 1990). 

Figure 3.68 Schematic of forward and reversed roll coating processes.

A great variety of techniques is available but as a basis for vinyl wallcoverings two only have become popular—reverse-roll coating and screen coating—both of which have the virtues of accuracy and high speed. 

Screen coating is described later in this chapter. The system used most widely is reverse-roll coating, which is capable of applying uniform coating of weights ranging from 80 to several hundred g m-2, within limits better than 5% and at speeds up to 150 m min-1. The usual form of the equipment is three-rolls, nip-fed, as illustrated in Figure 108. 

Fignre 108 Schematic representation of reverse-roll coating with three rolls... 

For the coating of continuous substrates, such as flooring, wall covering and tarpaulins, direct or transfer coating is used. The paste can be applied directly using knife on roller or belt, or indirectly via reverse roll coating. The substrate can be a carrier material which is removed later. This process permits multilayer... 

Further work is being directed to computer-aided analysis of (1) flow in other roll configurations, especially roll-wall combinations and reverse-roll coating (2) stability of the flow to meniscus nonuniformity — "ribbing," for example and (3) effects of viscoelasticity. 

Although virtually any type of roll coater may be used, reverse roll coating is the most common. This may incorporate a doctor roll, doctor blade, or Mayer bar to meter the spread rate. With tapes, adhesive is usually applied directly to the tape, which then passes through a drying station incorporating countercurrent air, usually warm. The dried tape is then rolled and slit. With label stock, adhesive is normally applied to the release paper, dried, and then transferred to the label when the label stock is united with the release paper in a nip roll. 

Reverse-roll coating n. A method of coating wherein the coating material is premetered between two rolls, one of which deposits the coating on a substrate. The thickness of the coating is controlled by the gap between the rollers and also by the speed of rotation of the coating roll. 

See also gravure coating, kiss-roll coating, and reverse-roll coating. 

Polyfunctional aliphatic isocyanate (Sumitomo Bayer Urethan Co., Ltd. Sumidur N NCO, 16.5X) was used as a cross-linker. Phosphate ester comprising mainly phosphorated polyoxyethylenated long-chain alcohols (Toho Chemical Industry Co., Ltd RL-210 mean molecular weight, 63O) was used as a surface active agent. Solutions in toluene of these samples were reverse-roll coated onto the film and dried in an air circulating oven at 90 C for 5 minutes. 

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