Polyamide/polyimide membranes

Polyamide/polyimide membranes these have greater stability to heat and chemicals, as well as better physical strength than the preceding type. Membranes made of nylon 66 are well known in winemaking. 

In recent tests (29), polyimide membranes showed higher rejection rates with hexane and isopropyl alcohol (IPA) than that of the polyamide membrane. The permeate flux of polyamide membrane with the crude cottonseed oil-hexane and IPA miscellas (20% by weight) were 2.25-5 times of the polyimide membrane (Table 13). The highest fluxes obtained with hexane and IPA misceUas were 6.6 1/m.hr and 1.4 1/m.h, respectively. 

Recent tests by Sun (32) have shown that polyimide membranes have higher rejection rates than those of polyamide membranes, but polyamide membranes have higher flux. The highest flux obtained with hexane miscella and polyamide membranes was 6.6 LMH. The phosphorus rejection rates of 98.1-99.3% were obtained with hexane miscella. The addition of surfactants increased the phosphorus rejection rate from 83.3-78.7% to 96.4% with IPA miscella. The added surfactants facilitated the formation of large phospholipid clusters. Koseoglu et al. (33) reported that membranes made of polyamide were least affected by hexane, but that a membrane made from a fluorinated polymer was deteriorated by hexane. 

Polymeric materials for MF membranes cover a very wide range, from relatively hydrophilic to very hydrophobic materials. Typical hydrophilic materials are polysulfone, poly(ether sulfone), cellulose (CE) and ceUiflose acetate, polyamide, polyimide, poly(etherimide) and polycarbonate (PC). Typical hydrophobic materials are polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE, Teflon) and poly(vinylidene fluoride). 

Typical solvents used in membrane production include N-methylpyrrolidinone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide, tetrahydrofuran, dioxane, dichloromethane, methyl acetate, ethyl acetate, and chloroform. They are used alone or in mixtures. These are used most frequently as non-solvents methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and t-butanol. Polymers involved include polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, polyolefins, polycarbonate, polyphenyleneoxide, poly(vinylidene fluoride), polyacrylonitrile, and cellulose and its derivatives. 

Commercial membranes for CO2 removal are polymer based, and the materials of choice are cellulose acetate, polyimides, polyamides, polysulfone, polycarbonates, and polyeth-erimide [12]. The most tested and used material is cellulose acetate, although polyimide has also some potential in certain CO2 removal applications. The properties of polyimides and other polymers can be modified to enhance the performance of the membrane. For instance, polyimide membranes were initially used for hydrogen recovery, but they were then modified for CO2 removal [13]. Cellulose acetate membranes were initially developed for reverse osmosis [14], and now they are the most popular CO2 removal membrane. To overcome state-of-the-art membranes for CO2 separation, new polymers, copolymers, block copolymers, blends and nanocomposites (mixed matrix membranes) have been developed [15-22]. However, many of them have failed during application because of different reasons (expensive materials, weak mechanical and chemical stability, etc.). 

By laminating conventional polyethylene and polypropylene microporous membranes together, it is possible to obtain a separator with the desired shutdown function together with protection from rupture. Microporous membrane of liquid crystalline polyester, polyphenylene ether, aromatic polyamide, polyimide, polyamide imide resin, acrylic resin, and cross-linked polymer are now being studied as candidates for lamination with polyethylene in order to gain even greater heat resistance. 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Membranes comprising silicone rubber coated onto polyimides, polyacrylonitrile or other microporous supports membranes are widely used [12,27]. Other rubbers such as ethylene-propylene terpolymers have been reported to have good properties also [28]. Polyamide-polyether block copolymers have also been used for pervaporation of some polar VOCs [29,30]... 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

A series of novel polyimides were prepared from diaminophthalide 288 and four anhydrides 289 for the investigation of water-permeable membranes. Of the polymer products 290, the best permeability was associated with bulky groups (trifluoromethyls) on the polyimide chains <2005JAPS2047>. There are other variants of diaminophthalide 288 used to prepare polyimides and polyamides. A fluorinated variation 291 has been used for polyimide synthesis <2004MI979> as has 292, derived from o-cresolphthalein 284 <1999PSA455>. 

Membrane asymmetric homogeneous, or microporous (cellulose acetate, polyamide, polysulfone, polyacrylonitrile) composite of a homogeneous polymer film on microporous substructure (cellulose acetate, polyamide, polysulfone, polyimide, polyvinyl alcohol). Usual pore 0.1 to 0.2 jm. 

Further developments are still being made in membrane materials and membrane modules. There are reports, in particular, of hydrophobic membranes made of polyolefins, crosslinked polyolefins, polyamides, polyaramids, poly(vinylidene fluoride), PTFE, polyimides, and suchlike. 

Du Pom, a leader in reverse osmosis technology built aronnd a unique class of tailored aromeik polyamides, was also an early leeder in the gas separation field.27,1 14,16 Molecuiariy engineered arometic polyimides were found by Du Pont to provide extraordinarily good flux and selectivity properties For hydrogen separations.27 Posttreataiem processes for these membranes were not reported. 

But of prime importance with regard to the final separation process is the nature of the membrane-forming polymer its hydrophihdty, charge density, polymer structure and molecular weight Typical polymers used in this phase-separation process are cellulose esters (most commonly CA), polyamides, poly(amide-hydra-zides), polyimides, (sulfonated) polysulfones, poly(phenylene oxide) and (sulfona-ted) poly(phthalazine ether sulfone ketone). 

Porous membranes can be made of polymers (polysulfones, polyacrylonitrile, polypropylene, silicones, perfluoropolymers, polyimides, polyamides, etc.), ceramics (alumina, silica, titania, zirconia, zeolites, etc.) or microporous carbons. Dense organic membranes are commonly used for molecular-scale separations involving gas and vapor mixtures, whereas the mean pore sizes of porous membranes is chosen considering the size of the species to be separated. Current membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), gas and vapor separation (GS), and pervaporation (PV). Figure 1 indicates the types and sizes of species typically separated by these different separation processes. 

Membrane permeation properties are largely governed by the pore sizes and the pore size distributions of UF membranes. Rather, thermal, chemical, mechanical, and biological stability are considered of greater importance. Typical UF membrane materials are polysulfone (PS), poly(ether sulfone), poly(ether ether ketone) (PEEK), cellulose acetate and other cellulose esters, polyacrylonitrile (PAN), poly(vinyKdene fluoride) (PVDF), polyimide (PI), poly(etherimide) (PEI), and aliphatic polyamide (PA). All these polymers have a Tg higher than 145 °C except for celliflose esters. They are also stable chemically and mechanically, and their biodegradabflity is low. The membranes are made by the dry-wet phase inversion technique. 

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