Evaporation rate polymer solutions

The heated polymer solution emerges as filaments from the spinneret into a column of warm air. Instantaneous loss of solvent from the surface of the filament causes a soHd skin to form over the stiU-Hquid interior. As the filament is heated by the warm air, more solvent evaporates. More than 80% of the solvent can be removed during a brief residence time of less than 1 s in the hot air column. The air column or cabinet height is 2—8 m, depending on the extent of drying required and the extmsion speed. The air flow may be concurrent or countercurrent to the direction of fiber movement. The fiber properties are contingent on the solvent-removal rate, and precise air flow and temperature control are necessary. 

Photoreceptors are prepared by the sequential application of the various layers onto a web or drum substrate. Vapor-deposition methods can be used for some pigments. Most layers, however, are coated from solution or dispersions in organic solvents. Wicks (1986) has reviewed film formation from polymer solutions. The choice of solvent is determined by such factors as solubility, evaporation rates, surface tension, toxicity, as well as environmental... 

The data can now be represented more conveniently in a triangular diagram, as in Fig. 12-2. This plot shows the approximate limiting solubility boundaries for polyfmethyl methacrylate). The boundary region separates efficient from poor solvents. The probable solubility parameters of the solute polymer will be at the heart of the solubility region. The boundaries are often of greater interest than the central region of such loops because considerations of evaporation rates, costs, and other properties may also influence the choice of solvents. 

Important disadvantages of this geometry are evaporation and free boundary effects for polymer solutions prepared with volatile solvents. Moreover, measurements are restricted to relatively low shear rates because polymer melts and other fluids will not stay in the gap at high rotational speeds. The cone-plate geometry is not recommended for measuring the viscosity of multiphase systems because in some cases domain sizes may be of the same order of magnitude as the gap size. 

When polymers crystallize from the melt or solution, the crystalline regions may exhibit various types of polymorphic modification, depending on the cooling rate, evaporation rate of solvent, temperature and other conditions. These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications have been reported (Tashiro Tadokoro, 1987). (See also Chapter 4.)... 

Several methods have been employed in the determination of the resistance of monolayers to evaporation. The evaporation rate may be determined from the increase in mass of a desiccant suspended over the monolayer, or from the loss of weight of a Petri dish containing solution and spread monolayer, under carefully controlled conditions. Such experiments are useful in determining the effect on permeability of incorporation of a plasticiser into the polymer stmcture. ... 

Polymer solutions allowed to evaporate produce polymeric films which can act as protective layers for tablets or granules containing sensitive dmg substances or as a rate-control-ling barrier to dmg release. Film coats have been divided into two types those that dissolve rapidly and those that behave as dialysis... 

Our screening concept is depicted in Fig. 19.2. For simplicity, operation of only one of the sensors from the sensor array is illustrated. An initially clean sensor crystal is exposed to a solvent containing a small amount of polymer followed by sensor withdrawal from the solution (Fig. 19.2a). Quantification of the residual dissolved polymer deposited onto the sensor is done by the measurements of the mass increase of the crystal, which is proportional to the amount of polymer film deposited onto the sensor from a polymer solution. Depending on the solvent-resistance of the polymers, the dissolution rate of polymers in solvents will be different as determined from multiple measurements of deposited polymers during the experiment (Fig. 19.2b). The measurements of frequency changes are done when the sensors are periodically withdrawn from the solvents and the solvents are evaporated. In this way, the measured signal change is indicative of the amount of the deposited material from the solution. 

Much has been written about ketones, as a class, as excellent solvents for vinyl polymers (7,8). Aside from solubility of the solvent, we also must consider its volatility characteristics (evaporation rate), flash point, boiling range, etc. With the high solubility type polymers, we have wide latitude with respect to selecting the ketone that will balance these properties against solution solids. This is evident in the next figures. 

Hansen (57) pointed out that evaporation of a solvent from a polymer solution faced two barriers when cast on an impermeable substrate resistance to solvent loss at the air-liquid interface and diffusion from within the film to the air interface. Evaporation of neat solvents as well as moderately dilute solutions is limited by resistance at the air interface, but as solvent concentration becomes low (5-10-15%), the rate-controlling step is diffusion through the film. Hansen pointed out that at the point when solvent loss changes to a diffusion-limited process, the concentration of solvent is sufficient to reduce the glass transition temperature, Tg, of the polymer to the film temperature. 

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