Coating methods solvent casting

Similar to screen printing, the spray coating method [95] is widely used for catalyst fabrication, especially in labs. The major difference between the two is that the viscosity of the ink for spray coating is much lower than that for screen printing. The application apparatus can be a manual spray gun or an auto-spraying system with programmed X-Y axes, movable robotic arm, an ink reservoir and supply loop, ink atomization, and a spray nozzle with adjustable flux and pressure. The catalyst ink can be coated on the gas diffusion layer or cast directly on the membrane. To prevent distortion and swelling of the membrane, either it is converted into Na+ form or a vacuum table is used to fix the membrane. The catalyst layer is dried in situ or put into an oven to remove the solvent. 

Numerous methods have been described in the literature for depositing coatings onto piezoelectric acoustic sensors. They generally fall into three categories solvent casting techniques, vacuum deposition techniques, and vapor-phase deposition techniques. 

In the case of porous solids, powders and fabrics, the measurement of contact angles is not an easy task. Any method is limited to the packing instability of the powders in the test medium, and all data should be viewed with caution, and whenever possible, the contact angles measured in compressed powder cakes should be confirmed by other independent measurements such as measurements on films of this substance prepared by solvent casting or dip coating. There are mainly two methods to measure the contact angle of solid powders ... 

Polymeric films can be prepared at the surface of metal, glassy carbon, as well as carbon paste electrodes. The preparation of conducting polymers at the surface of carbon electrodes employed in biosensors is already reviewed [1]. The methods mostly used are solvent casting, spin coating, and electropol5mierization. 

In solvent casting method an already prepared polymer is first dissolved in the appropriate solvent and then simply cast onto the surface of the electrode. After solvent evaporation, the film of polymer is formed, ft is a very simple approach, but unfortunately two disadvantages have to be considered, uniformity of the polymeric film and reproducibility of its preparation [2]. This method is usually used for the preparation of redox active or nonconducting polymers [3]. Coatings of composites of nanomaterials with polymers are also often prepared by this method [4]. 

Spin coating or spin casting is the preferred method for application of thin, xmiform films to flat supporting substrates. For this fabrication the conjugated polymer or its nanocomposite must be soluble or at least it should form fine suspension in common organic solvent. A schematic representation of spin-coating method is shown in Figure 12.13. 

Solvent Casting Techniques Solvent casting is perhaps the simplest coating method. It requires that the coating material be soluble in a solvent that does not chemically attack the piezoelectric sensor device and its transducers. Once the coating material is dissolved the solution can be spread over the device and the solvent evaporated to leave the desired coating material. Popular techniques in this category include [5, 19] ... 

Spin coating is a smaU-scale rapid method for ASD screening that requires minimum amounts of API. Sample preparation is simple and the technique is compatible with a number of analytical characterization methods. Automation of the process is possible but parallel processing obviously has certain Umitations. Similar to solvent casting approaches, the level of residual solvent after evaporation can be an issue, and the compounds need to dissolve and remain stable in the same volatile solvents (e.g., ethanol, acetone, etc.) as the polymers and/or other excipients used. 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

The application type defines the desired form of the synthetic material. A coating may be spin cast or solvent cast upon a substrate. The preparation of membranes is not trivial, as many techniques have been developed to give the desired properties methods include solution casting, melt pressing, and solvent precipitation. Detailed accounts of membrane preparation can be found elsewhere (J). Other material forms required by medical applications include powders (for bone cement) and films. This review will focus on applications that require injection molding of engineering thermoplastics. 

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