NS-100 Composite Membrane

The NS-100 membrane (initially designated as NS-1) was the first noncellu-losic composite membrane to appear in the published literature and have an impact on the reverse osmosis scene.22/23 This membrane, invented by Cadotte,24 consisted of a microporous polysulfone sheet coated with polyethylenimine, then interfacially reacted with either 2,4-toluenediisocyanate (TDI) or with isophthaloyl chloride (IPC). In the first case, a polyurea is formed in the second case, a polyamide. The chemistry of this membrane is as follows  

Strictly speaking, the descriptors NS-1 and NS-100 (North Star Research Institute) in the technical literature refer to the TDI-based polyurea membrane. MSI-400 (Membrane Systems, Inc.) refers to the same composition.25 PA-100 (UOP, Fluid Systems Division) and NS-101 have been used to name the corresponding IPC-based polyamide. The polyamide analog exhibits somewhat higher flux and slightly lower salt rejection than the polyurea form of the membrane. A low pressure version of PA-100/NS-101 membrane, optimized for brackish water operation on Mohawk-Wellton (Yuma) agricultural drainage water, was developed by UOP under the name TFC-202.26 

Polyethylenimine is a product of the self-condensation of the strained-ring compound ethylenimine (aziridine, azacyclopropane). It is a globular molecule having a not quite statistical distribution of amine groups (30% primary, 40% secondary and 30% tertiary amine). The most effective molecular weight range for this water-soluble polymer in composite membrane fabrication has been 10,000 to 60,000. 

The NS-100 membrane is capable of giving salt rejections in excess of 99% in tests on salt solutions simulating seawater (18 gfd, 3.5% synthetic seawater, 1,500 psi, 25°C). If the polyurea interfacial reaction step is omitted, and the polyethylenimine-coated polysulfone film is heat-cured as usual, a crosslinked polyethylenimine semipermeable barrier film is generated. This membrane gives 70% salt rejection and 55 gfd water flux under the same test conditions as above. Also, if the fully formed NS-100 membrane is dried at 75°C, which is too low a temperature to effectively crosslink the amine layer, the resulting film will exhibit a salt rejection of 96% or less. 

This membrane is highly sensitive to chemical attack by hypochlorite ion and hypochlorous acid in chlorinated feedwaters. Wrasidlo prepared a variation of the NS-100 membrane in which the primary amine groups of polyethylenimine were cyanoethylated before reaction with isophthaloyl chloride.27 This membrane was claimed to be chlorine-resistant, but was probably not so. 

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Figure 3b. SEM photomicrograph of composite membranes cross-section of a NS-100 composite membrane showing the porous polysulfone substructure.

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