Membrane casting dope

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

The viscosities of the membrane casting dopes were measured by a Hoeppler viscometer as previously described (6). The solution densities were obtained by the modified Gibson and Loeffler (,9) dllatometer. From these data the solution viscosities at different temperatures were calculated. The activation energies of viscous flow obtained from the logri-1/T plots were corrected by the corresponding values of the solvents used. 

The shape of the presented curve seems to be significant because an increase of the AR values is known (6,11) to indicate the enlargement of the supermolecular aggregation within the membrane casting dope. 

Further, the observed changes in trends of both the AE curve (Figure 1) and the membrane performance data (Figure 3) that happen in the same range of PA concentrations, lead to the conclusion that a certain stable level of supermolecular organization reached in the membrane casting dope creates conditions for the membranes with optimal skin properties to be formed. 

In order to explain the specific role of PA in the membrane casting dope one has to take into consideration the structure of PA. The molecule of PA forms (15,16) a tetrahedral POi, group which is linked to the other molecules by hydrogen bonds. In 86 wt.% PA two P0 groups are connected by one hydrogen bond (Figure 6) enabling formation of a chainlike structure of PA. 

The Effect of Phosphoric Acid as a Casting Dope Ingredient on Reverse-Osmosis Membrane Properties... 

Porous asymmetric CA membranes were prepared from all the solutions listed in Tables 1 and 2. Temperature of the casting solutions was 25°C except in one case which is later thoroughly discussed. The glass plate on which the solution was cast was kept at the same temperature as the casting dope. Temperature of the casting atmosphere was 20-25°C and the relative humidity was 60-65%. If not otherwise specified the evaporation period was as short as possible, i.e. the cast solution was immersed immediately into a gelation bath consisting of ice-cold water. 

These have been investigated on both the casting solution structure and the asymmetric membrane properties. Two relevant parameters have been examined 1. The PA concentration in the fixed amount of additive, i.e. the additive quality, and 2. the additive content in the casting dope. 

To elucidate this effect a series of membranes was prepared from the most promising casting dope (Batch 513) that has been cooled down to 0°C. The results presented In Figure 5 show that In this way a shift of the shrinkage temperature profile toward lower temperatures followed by the better membrane performance has been obtained. Such a result seems to be rather vmexpected, and It will be discussed later. 

Casting dope Bore fluid Precipitation bath Membrane type... 

In order to obtain in a very short time an optimum casting dope composition which produces a membrane of required performance, a guide line listed in Table II was established, based on our previous experience of membrane research. Mixtures of high and low boiling, water mlssible solvents for cellulose acetate were employed as solvents for dope. As for additives to the dope... 

In order to confirm that cyclohexanone is extracted more reluctantly than other solvents, the concentrations of extracted solvents in the gelation bath water were measured with immersion time lapse by using gas chromatography. As illustrated in Figure 12, the extraction rate of cyclohexanone is less than 1/3 of DMS s. In case of well-known acetone-formamlde casting dope for cellulose diacetate RO membrane, formamide is extracted faster than acetone when as cast membrane is immersed into chilled water... 

Figure 3.7s is a ternary phase diagram for a casting dope made of CA and acetone it shows several different paths of precipitation for porous UF membranes and dense RO membranes. Path AE produces an open UF membrane because water enters the film faster than the solvent leaves, and CA precipitates around a larger volume of solvent which acts as the "pore-former". Path AD leads to a tight RO membrane because solvent leaves the film faster than water enters. When CA precipitates, there is less volume of solvent present resulting in a lower porosity and smaller pores. 

Figure 3.9 Effect of water in casting dope on formation of UF membranes.

Figure 3.10 CA membrane porosity (water content) as a function of the solvent used in the casting dope.

The DMAc-cast membrane was doped with various concentrations (30, 40, 50, and 60wt%) of aqueous H3PO4 (PA) solution or 14(M) PA solution or 9(M) PA solution. Equilibrium was reached after 3 days at room temperature. The acid doping levels were expressed as moles of PA per mole of PBI repeat unit. Cross-linked PBI membranes require either covalently or ionically higher acid concentrations [97]. 

Casting by solvent evaporation is a commonly used procedure for fabrication of membranes based on organic polymers. It is probably the most widely used technique for polybenzimidazole membrane preparation for high-temperature polymer electrolyte membrane fuel cells. After casting, doping with phosphoric acid provides proton conductivity to the membrane. 

Proton conductivity phosphoric acid. Nation N117 and HjPO -PBI membranes as a function of temperature.The membrane prepared by the PPA process has an acid doping level of 32, and the conductivity was measured at 0% relative humidity the DMAc-cast and acid-imbibed membrane has an acid doping level of 5.7 (proton conductivity measurement in an atmosphere with water to air molar ration of 0.7) the membrane cast from trifluoroacetic acid has an acid doping level of 6.0 and the proton conductivity was measured in a relative humidity of 5%. (Source Li ef... 

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