Polypropylene Microfiltration Membranes Research Articles

Temperature-sensitive membranes prepared by UV photopolymerization of N-isopropylacrylamide on a surface of porous hydrophilic polypropylene membranes

Novel multifunctional membranes were prepared by ultraviolet photopolymerization of N-isopropylacrylamide (NIPAAm) on the surface of porous hydrophilic polypropylene microfiltration membranes. The poly N-isopropylacrylamide PNIPAAm gels were generated on the surface of the membrane through a covalent bond in the presence of a crosslinking agent, N, N′-methylenebisacrylamide. The crosslinked PNIPAAm gels are temperature-responsive hydrogels that can swell and deswell reversibly in aqueous solution around the vicinity of the lower critical solution temperature (LCST) of PNIPAAm. With a change of temperature, the effective pore size of the membrane surface can be enlarged or shrunk as the PNIPAAm gels swell or deswell. Above the LCST of PNIPAAm, the fluxes of water and solution containing 500 ppm of dextran (molecular weight: 1.67×10 5 g/mol) through the temperature-sensitive membranes are about 6 and 85 times higher than those below the LCST of PNIPAAm, respectively. The changeable flux makes it possible to employ the temperature-sensitive membranes as a sensor or valve that regulates filtration properties by responding to temperature. Solutions of dextran with a molecular weight from 6300 to 2,000,000 were used to evaluate the separation performance of the temperature-sensitive membranes as the ultrafiltration membrane. The ranges of rejection of dextran and the flux of solution are from 0 to 90 and 8 to 32 l/m 2 h, respectively, depending on the temperature, pressure, and molecular weight of dextran. It is clear that the temperature-sensitive membrane exhibits multifunctional characteristics; that is, the microfiltration membrane is above the LCST of PNIPAAm, and the ultrafiltration membrane is below the LCST of PNIPAAm.

Ultra-low pressure water softening: a new approach to membrane construction

The ultra-low pressure water softening properties of a series of novel membranes containing cross-linked poly(4-vinylpyridinium) salts incorporated into the pores of polyethylene or polypropylene microfiltration membranes have been examined. It has been shown that these membranes exhibit excellent separation properties with high permeabilities at low trans-membrane pressures. The separation mechanism in these pore-filled membranes is primarily due to Donnan exclusion, and as such bivalent cations are more strongly rejected than monovalent ions. Moreover, it has been shown that relatively large molecules such as sucrose are only rejected to a limited extent. The membranes have been shown to withstand free chlorine at ambient temperatures. The performance of the pore-filled membranes has been shown to compare favorably with existing state of the art thin-film composite (TFC) nanofiltration membranes (Osmonics Desal-51 and BQO1 and Hydranautics TFV-7450). In contrast to the TFC membranes, the separating layer in these pore-filled membranes has been shown to be thick, ranging from 80 to 160μm. These polyelectrolyte opre-filled membranes offer a new paradigm in the construction of high performance water softening membranes.

Acid recovery using diffusion dialysis with poly(4-vinylpyridine)-filled microporous membranes

Two series of membranes have been produced by photoinitiated polymerization of 4-vinylpyridine (4VP) and divinylbenzene (DVB) within the pores of polypropylene microfiltration membranes. The first series was comprised of membranes with varying mass gain and constant DVB content. The second series of membranes had similar mass gains but varying DVB content. The membranes were tested by diffusion dialysis of acid/salt solutions (HCl/NaCl/MgCl 2) in order to determine the effects of both mass gain and degree of crosslinking on dialysis coefficients and acid/salt separation. It was found for the first series of membranes that the dialysis coefficients of the acid and salts decreased and then leveled off with increasing mass gain while separation increased and then also leveled off. The second series of membranes showed a decrease in acid and salt dialysis coefficients but a dramatic increase in separation as the DVB content was increased. These results are interpreted in terms of the fixed charge concentration and the water content of the membranes. A comparison is made with a commercial diffusion dialysis membrane.

Development of a coating technique for the internal structure of polypropylene microfiltration membranes

A novel method of coating hydrophobic polyolefinic microfiltration (MF) membranes to produce a more hydrophilic membrane has been developed. A modified interfacial polymerization technique was used to coat the internal surface of a polypropylene (PP) membrane (about: 1.1 μm pore size, 84% void volume, 84 μm thick). 1,8-octanediamine (selected from several possible diamines) is dried onto the membrane internal surface from methanol and then reacted with a disulfonyl chloride (plus trisulfonyl chloride crosslinking agent) from a mixed solvent system of CHCl 3 and CCl 4, forming a polysulfonamide coating. Key polymerization parameters were identified as time and temperature of polymerization, concentrations of the diamine and the sulfonyl chlorides, and the ratio of CHCl 3 to CCl 4. The coating was uniform and stable. Permeation measurements were performed with various size polystyrene latex spheres and carboxylic modified polystyrene latex spheres in aqueous solution. Coating significantly increased hydrophillicity, and hence flux, and reduced membrane fouling for latex sphere solutions.

Poly(4-vinylpyridine)-filled microfiltration membranes: physicochemical properties and morphology

The morphology and physicochemical properties of poly(4-vinylpyridine)-filled microfiltration membranes have been examined. These membranes, which were prepared by photoinitiated grafting of up to 125 mass% of 4-vinylpyridine into the pores of polypropylene (PP) microfiltration membranes, were characterized in terms of the amount of poly(4-vinylpyridine) incorporated (graft yield), ion-exchange capacity, water content, and thickness. The morphology of samples of the grafted membranes dehydrated by freeze substitution was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. While the ion-exchange capacities of the grafted membranes are a function of graft yield, ranging to 4.0 meq/g of dry membrane, the water contents of the grafted membranes in the free base form are essentially independent of graft yield. The porosity of the grafted membranes was shown to be almost the same as that of the starting base polypropylene membranes (60–80%). However, protonation of the grafted poly(4-vinylpyridine) was shown to lead to a further and very substantial increase of the void volume of the membranes as measured by their water content. The thickness of the grafted membranes was found to increase linearly with increasing incorporation of polyvinylpyridine. Ionization of the polyelectrolyte was shown to cause a further systematic increase in thickness which was partly reversible with reversion to the unprotonated form. These changes in thickness are attributed to the stretching of the mesh of the substrate microfiltration membranes.

Porous, polyelectrolyte-filled membranes: Effect of cross-linking on flux and separation

A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine (4VP) with varying amounts of divinylbenzene (DVB) onto polypropylene microfiltration membranes. Using the same irradiation conditions, it has been found that graft yield increases substantially in the presence of DVB. The increase in graft yield was shown to be accompanied by a substantial increase in the thickness of the grafted membranes and a small but significant decrease in water content. The composite membranes have very high charge densities and good mechanical properties. The membranes with various amounts of DVB were quaternized (methylated) and examined for reverse osmosis of various salts. In addition to an expected drop in flux due to the increased thickness and decreased water content, there was significantly different salt rejection for membranes with cross-linking. While, for example, there is practically no difference in rejection of NaCl by a membrane with 0.55% DVB and one having no cross-linker, the Na 2SO 4 rejection by the cross-linked membrane is, on average, twice as high as that by the non-cross-linked one. Large differences between the cross-linked and non-cross-linked membranes were found in the ratios of pure water to NaCl permeate fluxes of the membranes at various pressures. The results are discussed in terms of the physicochemical nature of the membranes and conformational changes of the pore-grafted poly(4-vinylpyridine).

Photograft-polymer-modified microporous membranes with environment-sensitive permeabilities

Intraporous heterogeneous modification of commercial nylon (Ny) and polypropylene (PP) microfiltration membranes, both of 0.2 μm pore size, with grafted poly(acrylic acid) (g-PAA) was accomplished by first coating the membranes with a photoinitiator, benzophenone (BP), and then UV irradiation in acrylic acid (AA) solutions in water. The degree of modification (DG) depended strongly on AA concentration (c AA) and UV time. As estimated from PAA homopolymer GPC analyses, average degrees of graft polymerization between 680 and 2200 were achieved by varying c AA between 10 and 50 g/l. PP-g-PAA and Ny-g-PAA membranes were characterized with scanning electron microscopy, revealing outer and intraporous surface coverage with g-PAA. FTIR-ATR and energy-dispersive X-ray spectroscopy data verified the chemical composition and a gradient of DG over membrane thickness, but modification also of the bottom layer. Membrane swelling and permeabilities depending on DG and pH — above and below p K gPAA — were studied. PP-g-PAA membranes were almost dimensionally stable, and with intermediate DG (around 1 mg/cm 2) the ‘switch height’ of transmembrane permeability as function of pH was very high (by a factor of 100, even in 100 mM buffer). Ny-g-PAA membranes markedly changed the shape due to modification, swelling and additional pH changes; the mechanical stability — especially at DG > 1 mg/cm 2 — was poor, and the permeability response to pH was less pronounced. Thus, the differences in membrane material hydrophilicity caused different modification surface selectivity, which for Ny was reduced due to sorption of BP and AA during coating and polymerization, respectively. Two different types of graftpolymer-modified microporous membranes resulted: almost ‘perfect’ pore filling for PP-g-PAA, but simultaneous matrix and pore modification for Ny-g-PAA.

The influence of surface forces on the fouling of polypropylene microfiltration membranes

Abstract Although many factors influence the filtration of colloidal solutions using micro-porous membranes, it is possible to study some of the components by the selective control of filtration conditions. In this report, we have examined the effects of particle-particle and particle-membrane forces on the fouling of polypropylene micro-porous membranes, in aqueous solution. We have shown that both classical DLVO and solvation forces are required to explain fouling. In general, aggregation of the particles via either electrostatic or solvation forces increases the membrane flux, whereas attractive forces between particle and membrance fouling and reduce the flux, even when the suspended particles are aggregated.

A new class of polyelectrolyte-filled microfiltration membranes with environmentally controlled porosity

A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine onto polyethylene and polypropylene microfiltration membranes. By imposing the rigid structure of the polyolefinic substrate on the highly charged but extremely flexible and swellable gel of polyelectrolyte, stable membranes with very high charge density (> 3.5 mequiv/g) have been developed. The membranes show an outstanding pH valve effect and the capability of rejecting small inorganic ions in the process of reverse osmosis. The pressure-driven transport and separation with these membranes differ markedly from those of conventional reverse osmosis membranes being governed by the properties of the polyelectrolyte. Salt rejection depends strongly on the feed concentration, decreasing substantially with increase in concentration. The most characteristic features of separation with these membranes are higher rejection of polyvalent co-ions and lower rejection of polyvalent counter-ions compared to monovalent ions.

The use of hollow fiber cross‐flow microfiltration in bioaccumulation and continuous removal of heavy metals from solution by Saccharomyces cerevisiae

Cross-flow microfiltration was shown to retain Saccharomyces cerevisiae biomass utilized for heavy metal bioaccumulation. The passage of metal-laden influent through a series of sequential bioaccumulation systems allowed for further reductions in the levels of copper, cadmium, and cobalt in the final effluent than that afforded by a single bioaccumulation process. Serial bioaccumulation systems also allowed for partial separation of metals from dual metal influents. More than one elemental metal cation could be accumulated simultaneously and in greater quantities than when a single metal was present in the effluent (Cu(2+) 0.43 mmol, Cu(2+) + Cd(2+) 0.67 mmol, and Cu(2+) + Co(2+) 0.83 mmol/g yeast dry mass when the initial concentration of each of the metal species was 0.2 mmol x L(-1)). Co-accumulation of two different metal cations allowed higher total levels of bioaccumulation than found with a single metal. The flux rate was 2.9 x 10(2) L x h(-2)microm(-2) using a polypropylene microfiltration membrane (0.1 microm pore size) at 25 degrees C.