Stelio Munari

The formation of microporous polyvinylidene difluoride membranes by phase separation

Abstract Microporous polyvinylidene fluoride (PVDF) membranes were prepared by casting and coagulating solutions of the polymer in eight different solvents. The membranes were characterized by scanning electron microscope observations, water content measurements and water permeability tests. The possible correlations between the resulting membrane characteristics and the properties of both the polymer-solvent solutions and the solvent-nonsolvent systems are extensively discussed. The analysis indicates that most of the correlations suggested or adopted in the literature do not hold for PVDF membranes, whereas the transport properties of the solvent-nonsolvent system seem to affect significantly the final membrane structure and the resulting performances in separation processes.

ULTRAFILTRATION MEMBRANES - CHARACTERIZATION METHODS

Abstract The experimental data obtained by application of two different techniques for measuring the porosity of asymmetric ultrafiltration membranes are reported. The measurements were carried out keeping the membranes in conditions similar to those utilized in most ultrafiltration processes. The techniques used were: (a) cut-off evaluation by means of proteins of known molecular weight; (b) pore distribution measurement by means of the so-called “bubble pressure” method. It has been verified that use of these methods supplies information on the structure of asymmetric membranes and also on the influence of the operative parameters. p]Limitations for the reliability of above techniques were taken into account, namely in connection with chemical properties of the membrane and experimental conditions. From the above considerations it is inferred that all techniques must be uniform in order to compare experimental data from different laboratories.

High performance ultrafiltration membranes cast from LiCl doped solutions

Abstract This paper deals with the preparation and properties of ultrafiltration polyvinylidene fluoride based membranes obtained by casting and gelation of ternary solutions of polyvinylidene fluoride, solvent and LiCl as an appropriate inorganic salt additive. The membranes were tested in a laboratory scale pilot plant. Flux and rejection were evaluated as functions of the type of solvent and the amount of inorganic salt added. The influence of these latter variables on the morphology and mechanical properties of the membranes was also investigated. The role of LiCl on the properties and the mechanism of membrane formation is discussed.

Casting and performance of polyvinylidene fluoride based membranes

Abstract The investigation of factors governing membrane preparation is of paramount importance in order to understand the formation of the membrane and its behaviour under test conditions. A large amount of fundamental work on membrane formation has been carried out on cellulose acetate membranes, but less information is available for other polymers. The purpose of the present work is to provide a better understanding of the parameters governing the formation of polyvinylidene fluoride membranes. The parameters controlling the casting process have been investigated in some detail.

Permoporometric study on ultrafiltration membranes

Abstract Skin layer porosity of ultrafiltration membranes was evaluated by an instrument based on the combined bubble pressure and solvent permeability methods. Nuclepore membranes were first tested for calibration and reproducibility purposes. Measurements were then carried out both on membranes prepared in our labs and an commercial ones. Porosity results are discussed in relation to membrane preparation variables as well as to phenomena controlling membrane performance during ultrafiltration application.

Preparation and characterization of polysulfone-polyvinylpyrrolidone based membranes.

Abstract Microporous polysulfone membranes were prepared from casting solutions of polysulfone, solvent and polyvinylpyrrolidone as additive. The membranes were characterized by scanning electron microscope observations, porosity measurements and ultrafiltration tests. The effect of polyvinylpyrrolidone and its molecular weight on the viscosity of the casting solutions and the properties of the membranes is discussed.

Membrane morphology and transport properties

Abstract By changing properly the preparation conditions, it is possible starting from the same polymer to obtain asymmetric membranes with different morphology and U.F. performances. The SEM cross-section give us information about the overall structure. Unfortunately these results are not enough, as the membranes properties mainly depend from the thin dense skin layer poorly resolved with the SEM technique. On the other hand, rejection and fluxes investigated with the conventional U.F. pilot units only supply an indirect information about skin layer properties. With the purpose to improve the knowledge on this topics, we have developed an automatic porosimeter based on the combined bubble pressure and solvent permeability method in order to obtain the pore number and radius distribution of the membrane in wet conditions. Despite some problems related to the absolute values we have obtained some interesting results both for new and used membranes.

Ultrafiltration of hydrosoluble polymers. : Effect of operating conditions on the performance of the membrane

Abstract Two sulfonated polyvinylidene fluoride membranes of different porosity, prepared by the casting and gelation technique, were tested in an ultrafiltration laboratory-scale pilot plant with hydrosoluble polymer feed solutions. Polyethyleneglycols and dextrans of different average molecular weight were used as solutes for the feed solutions. Flux and rejection of the membranes were determined as functions of operating conditions (pressure, temperature and recirculation rate). The effect of properties of feed solutions (solute, concentration and molecular weight) on the performance of the membranes was also investigated.

Water—ethanol permeation through grafted PTFE membranes

Abstract A study is presented of water—ethanol permeation through grafted PTFE membranes containing small amounts of low-molecular-weight grafts. The permeation behaviour depends on conditioning time due to a rearrangement of amorphous and crystalline domains brought about by the permeating mixture. Selectivities observed are comparable to values reported for membranes with high-molecular-weight grafted chains.

Preparation and properties of membranes obtained by radiation induced grafting of 4-vinylpyridine onto polytetrafluoroethylene films

Abstract The preparation and properties of membranes obtained by radiation induced grafting are described, using vinylpyridine (VPy) on a polytetrafluoroethylene (PTFE) matrix. Subsequent quaternarization of the pyridine groups improves greatly the hyperfiltration properties of these membranes, considering both flux and rejection, and they may be developed for desalination purposes. The dependence of grafting yields on irradiation times was studied as well as the thermal and chemical properties of the membranes obtained. Flux and rejection to NaCl of quaternarized VPy/PTFE membranes was measured as a function of various chemical and physical properties.