Curtain-coating

Curtain coating is widely used in the coating of flat sheets of subsfrafe, such as wood panels. A coaling is pumped fhrough a slof in fhe coating head so that it flows as a continuous curtain of liquid. The maferial to be coated is moved under the curtain by a conveyor belt. The curtain is always wider than the substrate, and therefore a recirculating system is necessary for fhe overflow from the sides. 

The film thickness is controlled by the width of fhe slof and fhe speed of fhe substrate being coated the faster the speed, the thirmer is the coated wet film. Because there is no film splitting, fhe coating is laid down essentially smoofla.  

Unlike film and blade coaters, the application is contact-free. This considerably reduces the risk of web breaks and, therefore, improves runnability. It is expected that the significance of curtain coating will increase. 

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Adhesives (qv) used to make plywood are classified as either the exterior adhesive or the lesser quaHty interior adhesive. The terms relate to the abUity of the adhesive to survive exposure to moisture and weather. PhenoHc resins (qv) are commonly used as adhesives to make plywood. The difference between interior and exterior phenoHc resin adhesive is the filler level. Exterior rated plywood uses higher resin content adhesives. Interior rated plywood uses either highly extended (below 24% resin soHds) or protein-based adhesives. The adhesive is appHed to the veneer by roU coating, spraying, curtain coating, or foam extmsion. 

Air-bubble entrapment may occur in the case of a gravity-appHed continuous coating over an impermeable substrate. Bubbles may also be caused by moisture vaporization from the substrate. Remelting of the coating may minimize the bubble defects. Curtain coating equipment of this design is capable of operation at substrate speeds up to 500 m /min. 

S. E. Kistier and L. E. Scriven, "Finite Element Analysis of Dynamic Wetting for Curtain Coating at High Capillary Numbers," presented at... 

Curtain coating With this process, paint falls in a continuous curtain from a closely machined gap in a header tank on to the flat article passing below on a horizontal conveyor the excess paint is collected in the main tank and then passed up to the header tank. It is an ideal method of applying thicker coatings (60/im and above) to sheet metalwork. 

Curtain coating Only suitable for flat sheets of uniform dimensions On suitable articles this method will produce a uniform paint film providing the surface of the article is perfectly clean. 

A matter of considerable importance in the selection of an application method is its efficiency. Spray techniques are usually inefficient, since many droplets drift past the target and are lost. Even electrostatic spraying can waste as much as 35% of the paint. There is some loss of paint in most methods, but roller coating, curtain coating and electrodeposition are very efficient. Electrodeposition is also a very useful technique where corrosion resistance is important, since it applies a uniform coating over nearly all surfaces of even the most complex-shaped article. 

These coatings are applied by spray on more complex shapes, but on flat sheet or board roller coating is the preferred method, with curtain-coating used for thicker layers. 

Currie, J. N., 11 8 Curry powder, 23 166-167 Curtain coating, 7 20-21 method summarized, 7 5t shear rates, 7 32t Curtain spin process, 17 473-474 Curtius degradation, 2 573 Curved surgical needles, 24 206-207 Curve-fitting procedures, 14 237 Cusping, in fine art X-radiography,... 

FIGURE 18.16 Three principal industrial coatings processes for films and thin sheets (a) roll coating (b) blade coating and (c) curtain coating. 

Polymers are all around us. They serve as the very basis of both plant and animal life as proteins, nucleic acids, and polysaccharides. In construction they serve as the concrete, insulation, and wooden and composite beams. At home they are found as the materials for our rugs, pillows, curtains, coatings, wastepaper baskets, water pipes, window glass, and ice cube trays. In transportation they are present in ever-increasing amounts in our aircraft, automobiles, ships, and trucks. In communication they form critical components in our... 

Slide coating and curtain coating, schematically depicted in Fig. 3.69, are commonly used to apply multi-layered coatings. However, curtain coating has also been widely used to apply single layers of coatings to cardboard sheet. In both methods, the coating fluid is pre-metered. 

Various "chemical modifiers" may be added to the resin to improve certain features such as prepressing, dryout resistance or for specific application methods such as curtain coating where surface tension is important. 

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