Membrane polyamide

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

The formulator must be aware of the potential for binding when filtering protein solutions. Because of the cost of most protein materials, a membrane should be used that minimizes protein adsorption to the membrane surface. Typical filter media that minimize this binding include hydrophilic polyvinylidene difluoride and hydroxyl-modified hydrophilic polyamide membranes [17a]. Filter suppliers will evaluate the compatibility of the drug product with their membrane media and also validate bacterial retention of the selected membrane. 

The design and capacity of an RO unit is dependent upon the type of chemicals in the plating solution and the dragout solution rate. Certain chemicals require specific membranes. For instance, polyamide membranes work best on zinc chloride and nickel baths, and polyether/amide membranes are suggested for chromic acid and acid copper solutions. The flow rate across the membrane is very important. It should be set at a rate to obtain maximum product recovery. RO systems have a 95% recovery rate with some materials and with optimum membrane selection.22... 

In the early 1970 s, Bayer et al. reported the first use of soluble polymers as supports for the homogeneous catalysts. [52] They used non-crosslinked linear polystyrene (Mw ca. 100 000), which was chloromethylated and converted by treatment with potassium diphenylphosphide into soluble polydiphenyl(styrylmethyl)phosphines. Soluble macromolecular metal complexes were prepared by addition of various metal precursors e.g. [Rh(PPh3)Cl] and [RhH(CO)(PPh3)3]. The first complex was used in the hydrogenation reaction of 1-pentene at 22°C and 1 atm. H2. After 24 h (50% conversion in 3 h) the reaction solution was filtered through a polyamide membrane [53] and the catalysts could be retained quantitatively in the membrane filtration cell. [54] The catalyst was recycled 5 times. Using the second complex, a hydroformylation reaction of 1-pentene was carried out. After 72 h the reaction mixture was filtered through a polyamide membrane and recycled twice. 

All oxidants used must be removed in the final stage of the pretreatment process, as they are known to damage most polymer membranes used for desalination. In particular, chlorine is known to be harmful to commonly used thin-film composite polyamide membranes. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

A third relevant factor is the chemical composition of the membranes, with cellulose acetate, polyamide and a number of composite membranes sharing the seawater installed capacity. It is my estimate that polyamide membranes have at least 90% of the market. 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

Figure 8. SEM photomicrographs of a 0.45fiM polyamide membrane (a) surface at 1 (b) surface at 2 (c) cross-section with midline separation at 3.

As a result of prior field experience with the furan system, a qualifying test has been employed at Albany International Research Co. to measure the durability. Table III displays data of typical samples tested against synthetic seawater at 1000 psig maintained at a temperature of 50°C. Samples of cellulosic seawater membrane and polyamide membrane were found to fail in several hours of challenge by these conditions. 

The polyamide membranes prepared from aromatic diamines and aliphatic dichlorides, or from aliphatic diamines and aromatic dichlorides may be feasible for ultrafiltration as well as aliphatic polyamide membranes. Ohya 57) investigated the separation characteristics of asymmetric poly(xylyleneadipamide) (72) membrane under severe conditions of high temperature and high (or low) pH. 

Polybenzimidazolone membrane 21 developed by Teijin Ltd. had the following permeative characteristics Water permeation, 840 1/m2 - day salt rejection, 99.5% (1% NaCl aqueous solution, 80 kg/cm2)69). The membrane was less sensitive to plasticization with water than cellulose acetate and aromatic polyamide membranes... 

An oxidation-resistant polysulfone-polyamide membrane 22 was prepared by the reaction of equimolar amounts of 4,4 -diaminophenylsulfone and terephthaloyl chloride70). After soaking for 75 days at pH 1 2 in a 5 g/1 Cr03 solution, the membrane had a desalting ratio of > 99 % after a 260 hr continuous operation, while... 

Frost74) in Union Carbide Corporation also prepared polyether-polyamide membranes containing free carboxyl groups (24). The salt rejection of the membrane, however, was relatively low (46% for 3.5% NaCl aqueous solution at 70kg/cm2), although pure water flowed at 143 1/m2 day. 

Permeability of aromatic polyamide membranes have been improved by modification of aromatic rings with pendant polar groups, for examples sulfonic, carboxylic, carboxamide, and sulfonamide groups, in addition to the before-mentioned methoxy group. 

Heat-resistant polyamide membranes containing pendant sulfonamide groups were also fabricated 91 93). Thus the membrane prepared from 2,2 -disulfonamide-4,4 -diaminodiphenyl ether-isophthaloyl chloride copolymer 34 gave the water permeation rate of 1700 1/m2 day and salt rejection of 65 %. The film with 50 p... 

As described above, the modification of the polyamide membrane with pendant polar groups effectively improved their permeation characteristics. 

Only a few attempts were reported concerning the arrangement of MIP particles between two porous membranes, or their deposition on a single membrane. For example, Lehmann et al. used MIP nanoparticles with diameters between 50 nm and 300 nm imprinted with boc-L-phenylalanin-anilide obtained by miniemulsion polymerisation the selective rebinding properties as well as the hydrodynamic properties of the nanoparticles stacked between two polyamide membranes were studied [254]. 

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. 

R. McKinney and J.H. Rhodes, Aromatic Polyamide Membranes for Reverse Osmosis Separations, Macromolecules 4, 633 (1971). 

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