Adhesion reverse roll coating

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. 

This flexibility has proven invaluable in the design of coater-ready aqueous adhesives which are specifically designed for a wide range of application techniques. These techniques include reverse roll coating, curtain coating, gravure coating, and spray application. Key characteristics built into the aqueous adhesives to permit such a wide variety of application techniques include ... 

Using the hot melt approach, it is possible to mix and cast the adhesive film as a continuous operation. Here the individual raw materials are fed into a conventional screw extruder, are mixed under controlled temperature conditions and are then pumped out into a reverse-roll coating machine, which can produce film adhesives from as low as 50 g/m up to about 1800 g/mm. The schematic of such a process is shown in Fig. 55. 

For the highest viscosity adhesives reverse roll applicators can he used. Here the adhesive is pumped into the nip of the applicator rollers and the adherend to he coated is passed between a pressure roller and the metering roller. This allows an accurate coating of adhesive to he applied to the substrate. 

This procedure identifies plastisols that are spread-coated on different substrates such as paper, aluminum foil, wood, and plastic sheet or film. Application is by doctor blade, direct roll, or reverse roll operations (Chapter 10). As an example plastisols are roll coated on adhesively primed metal for house sidings, conveyor belts, cloth fabrics, wood panels, and so on. 

Adhesives for reverse roll application will typically have viscosities in the range of 1000 to 10,000 cP. While coating speeds in excess of 200m/min are possible, machine speeds are normally limited by the speed at which the film can be dried. A simple reverse roll coater is shown in Fig. 3. 

Solvent cements are usually supplied at concentrations of 10-25% solids, with viscosities in the range of 1,000-30,000 cPs. They may be applied by brush, spray, doctor blade, reverse-roll coater, hand roller, trowel, and many other application methods. The proper coating of the adhesive is obtained by establishing the correct viscosity for the particular application method. After application, the solvent is removed by ambient drying or by forced drying in heated ovens or tunnels. 

Self-Adhesive or Pressure-Sensitive Labels. It is preferable to call these pressure-sensitive labels, as both the pregummed and heat-sensitive labels are self-adhesive (i.e., the adhesive is already there). They consist of a suitable label facing material (usually paper or polymer), the reverse side of which is coated with a permanently tacky adhesive that is in contact with a backing paper (occasionally plastic) that protects it prior to use. The backing paper is coated with a special release coating that permits the label to be removed easily. Labels may be provided on roll or sheet form both can have the label laid on that is, the unprinted area has been cut and removed. 

Two-roll reverse For additional control over adhesive coating can handle wet film thicknesses from 1 to 20 mils... 

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