Polymeric amine reactants, membrane

Attempts to scale up the NS-300 membrane preparation from the laboratory scale to continuous machine production led to a high degree of variability in membrane properties ( ). The difference was attributed in part to the variability of machine-made polysulfone support film. Properties of the machine-made polysulfone supports differed from the laboratory cast support films. One of the major factors affecting this difference was that the machine-made support film was cast on a nonwoven polyester backing material which can vary in properties. Thus, the machine support film, which was quite adequate for NS-lOO type membranes using polymeric amine reactants, still remained a limitation for the monomeric amine reaction of NS-300. 

A broad-based effort in both the United States and Japan has since taken place to find other polymeric amine reactants that would contribute to better properties. Perhaps the most significant outcome of this effort to date has been the discovery of composite interfacial membranes based on "polyepiamine" by Wrasidlo,31 and their development into the PA-300 and RC-100 commercial forms by Riley and coworkers.32 33... 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

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