Doctor Blade coating method

Figure 6.12 The Doctor Blade coating method for the laminating of electrolyte and cathode membranes for LPB.

In knife-over-roll or blade coating, the coating material is placed on the fabric surface behind a knife, or doctor blade, and metered according to the gap set between the blade and the fabric surface. This method is used to apply thick coatings of highly viscous materials such as pastes, plastisols, or foams. 

Coating Doctor blade method Coating R Screen-printing method. 

Roller Coaters and Sheet Coaters. These arc often used in coil coating where very large, flat surface areas need to be painted quickly. In these methods, the objects to be painted are passed between a doctor blade and applicator rolls. These methods arc suitable only for coatings that arc to be baked. 

Methods which enploy liquid phase treating liquors (spray, transfer rolls and belts, etc.) share a cannon difficulty stemming from the lew volume of liquor in relation to the large surface area of the fibers conprising the substrate to be treated. This difficulty is acerbated when aqueous liquors are to be applied to hydrophilic fibers. Foam coating methods enploy stable foams which allow the thickness of the liquid-air mixture to be controlled by a doctor blade or roll. Conparatively low add-ons can be achieved by virtue of the low density of the foam layer, but the stable nature of such foam, and its immobile liquid phase, inhibit rapid, uniform distribution through the substrate. 

It is worthy of note that the thickness of the H-dip-coated film is much less than the gap height. This is characteristic of the main way in which the premetered H-dipping process differs from the conventional metered doctor-blade (or wire-bar) coating (Kuo et al., 2004). In the conventional approach, the doctor-blade (or wire-bar) coating process produces a film thickness of the order of the gap size whose thickness is independent of the carrying sp>eed of the substrate. In the H-dipping method, the premetered process allows the critical control of the thickness and can produce superior quality and extremely thin films at line speeds of the order of a few meters per minute. 

Dip Coating - This method is the most popular way to coat cloth and fibers with polytetrafluoro-ethylene dispersion. Typically, the substrate is dipped in the dispersion and excess dispersion is removed by a device such as a doctor blade. The wet coated substrate is then further processed. Viscosity of the dispersion determines the initial thickness of the wet coating immediately after removal from the dip tank. 

The method used for the preparation of thin films of PPV or its derivatives depends on the specific potymer. For those that present high solubility in common organic solvents, very simple procedures, such as casting, doctor-blade technique [98], or spin-coating of the polymer solution with further solvent evaporation can be used. In the case of spin-coating, the film morphology is sensitive to the solvent evaporation rate, density, viscosity, polarity, and solubility during the process [99]. 

Aside from the above process, a decal transfer method has also been developed to make a thin-film electrode [81-83]. The catalyst ink is first coated onto a decal substrate (such as PTFE film or Kapton film) by spraying or by using the doctor blade technique. The catalyst ink is then transferred to a Nafion membrane by a hot-pressing process to form a catalyzed membrane. This decal transfer method is presented schematically in Fig. 2.15 [83]. 

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. 

---