Composition of the casting solution

In most of the examples discussed so far the casting solution has consisted solely of polymer and solvent. However, the addition of nonsolvent has a considerable effect on the membrane structure. The maximum amount of nonsolvent that can be added to a polymer solution can be deduced from the ternary diagram, in the same way as the case of the maximum amount of solvent which can be allowed in the coaguladon bath. The only requirement is that no demixing may occur, which means that the composition must be in the one-phase region where all the components are completely miscible with each othet 

On adding nonsolvent to a polymer solution, the composition shifts in the direction of the liquid-liquid demixing gap. In this case, figure III - 49 illustrates the calculated 

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Fig. 6. Phase diagram showing the composition pathway traveled by the casting solution during precipitation by cooling. Point A represents the initial temperature and composition of the casting solution. The cloud point is the point of fast precipitation. In the two-phase region tie lines linking the...

Membrane filters are usually made by casting a polymer solution on a surface and then gelling the liquid film slowly by exposing it to humid air. The size of the pores in the membranes can be varied by altering the composition of the casting solution or the gelation condition. Another common technique is to irradiate a thin polymeric film in a field of a-particles and then chemically etch the film to produce well-defined pores. 

Other preparation parameters are polymer concentration, evaporation time, humidity, temperature, and the composition of the casting solution (e.g. additives). These parameters are mainly determining the ultimate membrane perfonnance (flux and selectivity) and hence for its application. The relation between these parameters and membrane structure will be described in greater detail in section IH - 6. The membranes obtained after precipitation can be used directly or a post treatment (e.g. heat treatment) can be applied. 

As an alternative to the PES membranes, cellulose acetate (CA with 39.8% acetyl content) membranes (with a thickness of 25 [im) were also prepared by solvent casting method. The composition of the casting solution was 2% CA (w/v) in acetone. Film formation was achieved by evaporation of acetone at room temperature (20 C) in 80% relative humidity environment. 

Ductility Tests. The ductihty of plated metals differs considerably from the corresponding thermally cast metals. Additionally, ductihty which is an important property if parts are to be deformed after plating, varies with the chemical composition of the plating solution, as well as the operating conditions of a given plating process. Ductihty can also be important when plated parts are stressed in use. Some metal deposits have coefficients of... 

Effect of Casting Solution Composition on the Casting Solution and Membrane Properties... 

Polysulfone membranes were prepared from 12.5, 13.75, and 15% (wt. %) polysulfone solution in dimethylformamide and formed on the surface of porous, sintered polymethyl methacrylate bars. An effective surface of each membrane was 49.2 cm. The effect of some casting parameters (composition and the temperature of the casting solution, time of solvent evaporation) and the pressure applied on the transport and separation properties of the membranes were analyzed. The experiments were carried out in a 1.2 dm pressure apparatus with continuous circulation of the permeate between feeding tank and the apparatus. It was found that membranes cast from 12.5% polysulfone solution of a temperature of 298 K with no solvent evaporation displayed the best properties. After 160 hours of operation at 0.18 MPa, the membranes in question showed an ability of a 97 to 99% rejection of 781.2 molecular-weight dye. The volume flux of the dye solution varied from 0.6 to 0.8m /m per day. 

Espinoza-Gomez and Lin prepared negatively charged hydrophilic UF membranes from the blend of acrylonitrile-vinyl acetate (CP16) and acrylonitrile-vinyl acetate-sodium p-sulfophenyl methallyl ether (CP24) [49]. The compositions of membrane casting solutions for nine different membranes are summarized in Table 5.17. 

Composition (wt%) of the Casting Solutions, Identification Codes, and Morphological Properties of PVDF Membranes... 

The extension and size of the porous fraction of a membrane depend substantially on the demixing rates, which can vary between the surface and bulk of the casting solution depending on the solution composition. At the upper surface the precipitation is practically instantaneous. This produces a barrier which slows the velocity of solvent/non-solvent exchange and reduces the precipitation rate from the upper to the bottom surface. Consequently a longer phase separation is produced in the bulk, leading to an increase in the average pore size. 

The phase diagram in Figure 12 shows the precipitation pathway of the casting solution during membrane formation. During membrane formation, the system changes from a composition A, which represents the initial casting solution... 

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). 

Immobilization of the bilayer membranes as thin solid films is required when the bilayer membranes are used as novel functional materials. Casting method is a simple way to immobilize the bilayer membrane on a solid support from an aqueous solution by drying. Polymer film is easily prepared when the cast film of polymerizable bilayer membrane is polymerized. A free standing polymer film prepared by photo polymerization of the cast film of diacetylene amphiphiles was reported by O Brien and co-workers [34]. Composition with macromolecular materials is another way of polymer film preparation. Bilayer membranes are immobilized as polymer composites by the following physical methods ... 

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