Aromatic interfacial composite membrane

Cellulose acetate and linear aromatic polyamide membranes were the industry standard until 1972, when John Cadotte, then at North Star Research, prepared the first interfacial composite polyamide membrane.8 This new membrane exhibited both higher throughput and rejection of solutes at lower operating pressure than the here-to-date cellulose acetate and linear aromatic polyamide membranes. Later, Cadotte developed a fully aromatic interfacial composite membrane based on the reaction of phenylene diamine and trimesoyl chloride. This membrane became the new industry standard and is known today as FT30, and it is the basis for the majority... 

Nonetheless a few commercially successful noncellulosic membrane materials were developed. Polyamide membranes in particular were developed by several groups. Aliphatic polyamides have low rejections and modest fluxes, but aromatic polyamide membranes were successfully developed by Toray [25], Chemstrad (Monsanto) [26] and Permasep (Du Pont) [27], all in hollow fiber form. These membranes have good seawater salt rejections of up to 99.5 %, but the fluxes are low, in the 1 to 3 gal/ft2 day range. The Permasep membrane, in hollow fine fiber form to overcome the low water permeability problems, was produced under the names B-10 and B-15 for seawater desalination plants until the year 2000. The structure of the Permasep B-15 polymer is shown in Figure 5.7. Polyamide membranes, like interfacial composite membranes, are susceptible to degradation by chlorine because of their amide bonds. 

The chemistry and properties of some of the important interfacial composite membranes developed over the past 25 years are summarized in Table 5.1 [10,12,29,30], The chemistry of the FT-30 membrane, which has an all-aromatic structure based on the reaction of phenylene diamine and trimesoyl chloride, is widely used. This chemistry, first developed by Cadotte [9] and shown in Figure 5.9, is now used in modified form by all the major reverse osmosis membrane producers. 

For a few years after the development of the first interfacial composite membranes, it was believed that the amine portion of the reaction chemistry had to be polymeric to obtain good membranes. This is not the case, and the monomeric amines, piperazine and phenylenediamine, have been used to form membranes with very good properties. Interfacial composite membranes based on urea or amide bonds are subject to degradation by chlorine attack, but the rate of degradation of the membrane is slowed significantly if tertiary aromatic amines are used and the membranes are highly crosslinked. Chemistries based on all-aromatic or piperazine structures are moderately chlorine tolerant and can withstand very low level exposure to chlorine for prolonged periods or exposure to ppm levels... 

A typical recipe for an interfacially formed aromatic polyamide composite membrane comprised a 2.0% aqueous solution of the aromatic diamine and a 0.1% nonaqueous solution of trimesoyl chloride. This recipe was extraordinarily simple, and ran quite contrary to experience with piperazine-based membranes. For example, surfactants and acid acceptors in the aromatic diamine solution were generally not beneficial, and in many cases degraded membrane performance by lowering salt rejection. In contrast, surfactants and acid acceptors were almost always beneficial in the NS-300 membrane system. In the nonaqueous phase, use of isophthaloyl chloride as a partial replacement for trimesoyl chloride had relatively little effect on flux, but tended to decrease salt rejection and increase susceptibility to chlorine attack. 

Homogeneous asymmetric CAs and polyamides made by the phase inversion process and cross-linked TFC polyamides have been the workhorse of RO plants for more than 30 years [21], Both CA and PA membranes possess an economically viable combination of high rejection and water flux [8]. However, TFC membranes now dominate the RO/NF market with CA membranes a distant second. For example, with the exception of Toyobo CTA polymer, all new seawater RO desalination plants deploy interfacial composite membranes of the fuUy aromatic type manufactured by Dow, Hydranautics (Nitto Denko), Tri-Sep and Toray. Along with the ability to remain stable over a greater pH range than cellulose-based membranes, TFC membranes exhibit much higher intrinsic water permeabilities because of their extremely thin ( 100 nm) polyamide-selective layers [21]. A typical spectrum of TFC membranes for various applications is given in Table 1.8. 

Nanofiltration membranes usually have good rejections of organic compounds having molecular weights above 200—500 (114,115). NF provides the possibility of selective separation of certain organics from concentrated monovalent salt solutions such as NaCl. The most important nanofiltration membranes are composite membranes made by interfacial polymerization. Polyamides made from piperazine and aromatic acyl chlorides are examples of widely used nanofiltration membrane. Nanofiltration has been used in several commercial applications, among which are demineralization, oiganic removal, heavy-metal removal, and color removal (116). 

As shown above, aromatic rings are connected by an amide linkage, -CONH-. While the aromatic ring attached to -NH- is m a-substituted, the ring attached to -CO- is the mixture of meta- and para-substitutions, which gives more flexibility to the polymeric material. Aromatic polyamide remains one of the most important materials for RO membranes because the thin selective layer of composite membranes is aromatic polyamide synthesized by interfacial in situ polymerization. 

Cadotte discovered that aromatic diamines, interfacially reacted with triacyl halides, gave membranes with dramatically different reverse osmosis performance characteristics than membranes based on aliphatic diamines. 56 Before that time, the area of aromatic amines in interfacial membrane formation had been neglected because of two factors (a) the emphasis on chlorine-resistant compositions, which favored use of secondary aliphatic amines such as piperazine, and (b) poor results that had been observed in early work on interfacial aromatic polyamides. The extensive patent network in aromatic polyamide (aramid) technology may also have been a limiting factor. 

Figure 5.11 SEM photographs of the surface texture of composite polyamide membranes from aliphatic and aromatic amines (a) uncoated microporous poly-sulfone (b) polyamide from polyethylenimine and trimesoyl chloride (c) tri-ethylenetetramine and trimesoyl chloride (d) 1,3-benzenediamine and trimesoyl chloride (e) 2,4-toluenediamine and trimesoyl chloride (f) 4-methoxy-1,3-benzenediamine and trimesoyl chloride. Note the smooth surface for aliphatic amine-based interfacial trimesamides and the coarse ridge-and-valley structure for aromatic amine-based interfacial trimesamides.

Kwak and Ihm [7] used AFM and solid state NMR spectroscopy to characterize structure-property-performance correlations in high-flux RO membranes. The membranes were thin film composites, whose thin active layers were based on aromatic polyamide formed by the interfacial polymerization of MPD and trimesoyl chloride (TMC). These membranes, each coded as SH-I, SH-II, and SH-III, were provided by Saechan (Yongin-city, Korea). The variations among these commercial membranes are difficult to know. Most likely, they vary by the amount of catalyst or surfactant added to the aqueous MPD solution. Table 8.2 shows water flux, salt rejection, and the roughness parameter of those membranes, together with the data for another membrane, MPD/TMC, which was prepared at the laboratory of Kwak and Ihm [7]. 

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