Solution-polymerized wiping

In commercial practice, when a volatfle component must be extracted from a polymeric solution of high viscosity, say 500 P or greater, the extraction operation is almost always conducted in geometries which mechanically generate a wiped film. And here, screw extruders have played a major role. 

The second effect which results from the cross-channel component of the barrel motion is the generation of a wiped film of the polymeric solution as the solution is dragged from the nip in an adjacent screw channel through the clearance between the flight tip and the barrel. Since this film is continually generated, mass is transferred to the gas phase in a time period given by... 

Finally, a number of experimental studies have been conducted in a pressure range where the polymeric solution could boil. The vapor bubbles thus created would provide a much larger surface area for mass transfer than the surface area of the wiped film alone. And therefore, for fixed values of the diffusivity and the driving force, predicted values for mass transfer rates would be substantially lower than the measured values. Conversely, for a fixed mass transfer rate and driving force, use of the wiped film surface area alone would require unusually high values of the diffusivity in order to obtain agreement between theory and experiment. 

Mass transfer in polymeric solutions by molecular diffusion is a comparatively slow process, and in extraction equipment where mass transfer occurs through a thin wiped film, inordinately large equipment surface areas are often required in order to obtain substantial rates of mass transfer. In commercial practice when this situation occurs, the required surface areas may, instead, be obtained by either one of two methods, both of which involve reducing the pressure in the extraction zone. In one approach the extraction pressure is fixed at a value which is less than the equilibrium partial pressure of the monomer or solvent in the polymeric solution fed to the extraction zone. In these circumstances gas bubbles... 

Fig. 10 a Example of polymeric nano-structure deposited from solution by mechanical wiping on a glass surface (red circles) compared to the untreated glass surface (blue crosses) as measured with the ultra-high resolution set-up. The corresponding resolution limit is marked by a vertical line and the most prominent in-plane length of 7.0 pm is marked by an arrow. The solid lines are fits based on a model described in the text, b Optical micrograph of the nanostructures... 

Migrating antistats are satisfactory solutions in many cases, but inherendy dissipating polymers (IDPs) or conductive polymers added to POs induce permanent antistatic/ESD properties. Two main advantages of these materials are usually dted in the literature (1) IDPs are large molecules that are not consumed, so they provide stable antistat/ESD properties over time and (2) IDPs do not bloom to the surface, so they cannot be "wiped off" and do not affect other surface properties. They are also usually clear, thermally stable, and some are much less affected by humidity than migrating antistats. Permanent polymeric antistats are usually conductive block copolymers that form continuous conductive networks when added to a polymer matrix. Many IDPs are not compatible with POs the ones discussed below are compatible [6-16,6-20, 6-28). 

Selection of the solvent should be dependent on the plastic being cleaned. Certain solvents will aggressively attack certain polymeric surfaces, causing the surface to deform or exhibit stress crazing. Substrates can be wiped or sprayed with degreasing solvents, or they can be immersed in an agitated solution for about 5 min. 

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