Coating solutions

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Air-Suspension Coa.ting. The Wurster process utilizes a cylindrical chamber in which the cores are suspended in a controlled stream of air. Film coatings are appHed by introducing the coating solution into the airstream, where the solvent evaporates quickly. The process is much quicker than film coating however, care must be taken to avoid destmction of the cores by attrition in the air stream. 

Coating solutions often exhibit a mixture of viscous and elastic behavior, with the response of a particular system depending on the stmcture of the material and the extent of deformation. Eor example, polymer melts can be highly elastic if a polymer chain can stretch when subjected to deformation. 

The behavior of colloidal suspensions is controlled by iaterparticle forces, the range of which rarely extends more than a particle diameter (see Colloids). Consequentiy suspensions tend to behave like viscous Hquids except at very high particle concentrations when the particles are forced iato close proximity. Because many coating solutions consist of complex mixtures of polymer and coUoidal material, a thorough characterization of the bulk rheology requires a number of different measurements. 

Dip and Spin Coating. The dip coating technique described for webs can also be used to coat discrete surfaces such as toys and automotive parts. The surface to be coated is suspended on a conveyor and the part dipped into the coating solution. The surface is then removed, the coating drains, then levels to give the desired coverage. The object is then dried or cured in an oven. 

Kapton resin (5) (113) and Upilex (6) resin (114) are sold as films, partially polymerized resin coating solutions, and sinterable powders. Allied Signal also offers Apical polyimide film. 

Phosphate coating solution Accelerator Immersion lime (min) Type of coating Coating weight (g/m )... 

Dip the silver electrode in the coating solution to a depth of 10 mm up the side of the glass tube. Allow to dry for 30 min and repeat twice. 

Figure 11 Ratio of osmotic to diffusional release rates plotted versus the glycerol content in the coating solution. (From Ref. 20.)...

Asymmetrical membrane capsules were prepared, for example, by dip coating mandrels with solutions containing 15 wt% CA and 33 wt% ethanol in acetone. After the mandrels were withdrawn from the coating solution they were immersed in water to precipitate the polymer and create the asymmetrical membrane capsule shell. This process was used to create both the body and the cap for the capsules. The capsules were sealed at the juncture between the cap and body by banding with a solution of CA in acetone. 

After the cleaning process, other techniques are used to prepare the surface of the substrate for coating. Some techniques include drying, surface etching, and chemical surface preparation. Examples of chemical surface preparation include the formation of an oxide layer or the monolayer assembly of an adhesion promoter on the surface. These processes modify the surface of the substrates so as to facilitate the subsequent deposition process. In surface preparation, frequently, the hydrophilic/hydrophobic character of the surface is controlled to match the coating solution properties. For example, Van Driessche et al.19 reported on improving the wettability of Ni-4at%W tapes... 

Spray 2nm-20pm Fast and adaptable to complex shapes and sizes, conformal step coverage, high efficiency Expensive, low- viscosity coating solution Electrical insulation, circuit board housing... 

In this paper the planarizing properties of some commercially available resins and monomers are evaluated. Other important properties such as etching resistance, film absorbance and glass transition temperature Tg are reported and discussed. Some of the materials that we evaluated are not marketed for use in the microelectronics industry. Consequently, they are not available as filtered spin coating solutions and may contain high levels of metal impurities that adversely affect device performance. 

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