Roll coating

Roll coating (Fig. 11-8) is the second most important method for application after spraying. The leather passes between two rolls (color print roll and transportation roU) by means of a transportation belt. The top roll transfers the relatively viscous color to the leather. The texture and the direction of rotation (synchronous or reverse) of the color print roU determine the amounts applied. Soft leathers can be processed on this machine only if the rolls turn in the same direction. Consequently, these types of leather can be roU coated only with finishes that do not require high amounts. 

In principle, roll coating is very similar to calendering, the difference being that the fluid does not separate from the roll. The fluid continues adhered to both surfaces, the substrate or web and the roll. In the analysis presented below, we follow the model presented by Middleman [16]. His derivation assumes that the coating fluid evenly wets both surfaces as schematically depicted in Fig. 6.38, as well as a Netwonian, isothermal and steady state flow. 

Using these assumptions and the notation presented in Fig. 6.38 the equation of motion reduces to 

At this point, the pressure and pressure gradient are unknown. However, we can perform a mass balance by using eqn. (6.186) to compute the volumetric flow rate 

Middleman suggests the simplest model for the separation region based on the assumption that the fluid splits at a point where ux = 0 and p = 0. The above equation can be solved numerically to give 

For a power law solution the equation, of motion is non-dimensionalized in a similar manner as with the Newtonian solution, except that dimensionless pressure is defined as 

In the process of roll coating, a sheet is produced much as in calendering but usually for polymeric solutions. In such flows, surface tension becomes important. Both forward and reverse roll-coating operations are used in practice. The important work by Coyle et al. [84—86] has done much to increase our understanding of the fluid dynamics in both forward and reverse roll coating. The reverse process is schemat- 

When multiple coats of an adhesive are required, the most uniform film thickness is achieved by applying the second coat perpendicular to the first. In addition, the time between successive coats must be carefully regulated. 

Too short a drying time may result in sagging, bubbling, or blistering, while too long a time may lend to the lifting of earlier coats. 

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The viscosity of elastomeric adhesives determines their method of application. Typically, solvent-borne rubber compounds require low viscosities for application. Thus, viscosities lower than 0.6 Pas for spray, curtain or dip applications are required, whereas for brush application viscosities lower than 5 Pa s are generally used. Roll- coating technology requires viscosities between 5 and 15 Pa s. 

Enamels. Eight commercial enamels, listed in Table I, were applied by roll coating to panels (25 X 76 cm) of 43 kg, Type MR-TU, No. 25 electrolytic tinplate. These panels were cut into strips (10 X 25 cm) and tested for flexibility before and after irradiation with the General Electric impact apparatus by the reverse impact method. The impactor... 

Roll coating of paint is the final process in a coil coating line. Roll coating is an economical method to paint large areas of metal with a variety of finishes and to produce a uniform and high-quality coating. The reverse roll procedure for coils is used by the coil coating industry, and allows both sides of the coil to be painted simultaneously. 

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. 

Continuous coating can be obtained by roll coating or spreading. There are three main versions ... 

Jian-hua et al. [137] studied how the MPL sintering temperature and time affected the performance of a fuel cell. It was observed that the best performance was obtained when the MPL was sintered at 350°C for 30 minutes. Lee et al. [138] demonstrated that DLs with an MPL that was screen printed or sprayed performed substantially better than when the MPL was roll coated. In general, the spraying and screen-printing methods are the most widely used. 

Uses Solvent for nitrocellulose, gums, and resins roll-coating inks, varnishes, lacquers, stains, and enamels starting material for synthesizing methyl isobutyl ketone insect repellent ore flotation. 

Roll coating is widely used for uniform, whether flat or cylindrical, surfaces including fiber bundles. In a roll coating process, fibers are coated between two rollers, an applicator roller and a backup roller coating is fed continuously to the applicator roller by a feed roller which runs partially immersed in a coating bath and the backup roller pulls the fibers by rotating in opposite directions. Slow... 

Wire is coated by being passed through a plastic extruder, but most materials are coated with solutions, emulsions, or hot powders. The classic brushing process has been replaced by roll coating, spraying, and hot powder coating. The application of polymers from water dispersions to large objects, such as automobile frames, has been improved by electrode-positon of the polymer onto the metal surface. 

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

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. 

Different roll coating methods and roll arrangements S = steel roll, R = rubber-covered roll, E = engraved roll. (Courtesy of Faustel Corporation.)... 

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

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