Membrane Technology

Definition of membrane

Membrane is a selective barrier between two phases.

Classification of membrane

·         Nature :

1.      Biological membranes :

o   living membrane

o   non-living membrane (liposomes and vesicles from phospolipids)

2.      Synthetic membranes :

o   Organic membrane (polymeric, liquid)

o   Inorganic membrane (ceramic, metal)

·         Morphology or structure

1.      Symmetric membrane :

o   Cylindrical porous

o   Porous

o   Homogenous (non porous)

2.      Asymmetric membrane :

o   Porous

o   Porous with toplayer

o   Composite

Preparation of synthetic membrane

Three basic types of membranes can be distinguished based on structure and separation principles:

·         Porous membranes

Membranes of this class induce separation by discriminating between particle size. Such membranes are used in microfiltration and ultrafiltration.

·         Nonporous membranes

Membranes from this class are capable of separating molecules of approximately the same size from each other. Separation takes place through differences in solubility or in diffusivity. Such membranes are used in pervaporation, vapour permeation, gas separation, and dialysis.

·         Carrier membranes

Using a very specific carrier-molecule which facilitates specific transport. Two different concept: the carrier is fixed to the membrane matrix or the carrier is mobile when it is dissolved in a liquid.

All kinds of different synthetic materials can be used for preparing membranes. The materials can either be inorganic (ceramic, glass, metal) or organic (polymers).

The most important techniques to prepare synthetic membranes, such as:

·         Sintering

This methode involves compressing a powder consisting of particles of a given size and sintering at elevated temperatures. The required temperature depends of material used. Materials can be used such as powders of polymers (polyethylene, polytetrafluoroethylene, polypropylene), metals (stainless steel, tungsten), ceramics (aluminium oxide, zirconium oxide), graphite (carbon), and glass (silicates). Pore sizes about 0.1 to 10 µm. Porosity 10-20% or higher.

·         Stretching

An extruded film or foil made from a partially crystalline polymeric material (polytetrafluoroethylene, polypropylene, polyethylene) is streched perpendicular to the direction of the extrusion, so that the crystalline regions are located parallel to the extrusion direction. Pore sizes 0.1-3 µm. Porosity higher than membranes by sintering, values up to 90% can be obtained.

·         Track-etching

A film or foil (often a polycarbonate) is subjected to high energy particle radiation applied perpendicular to the film. The particles damage the polymer ,matrix and create tracks. The film is then immersed in an acid or alkaline bath and the polymeric material is etched away along these tracks to form uniform cylindrical pores. Pore sizes 0.02-10 µm. Porosity max 10%. Energy of particles usually 1 MeV.

·         Template leaching

Leaching out one of the components from a film. A homogenous melt (1000-1500⁰C) of a three component (Na₂O-B₂O₃-SiO₂) system is cooled to separate the system into two phases. One phase consists mainly SiO₂ which is not soluble. The second phase is soluble and leached out by an acid or base and a wide range of pore diameters can be obtained with min size 0.005 µm.

·         Coating

Dense polymeric membranes in which transport takes place by diffusion generally show low fluxes. To increase the flux through these membranes, the effective membrane thickness must be reduced as much as posible. This may be achieved by preparing composite membranes.

Composite membranes consists of two different materials, with a very selective membrane material being deposited as a thin layer upon a more or less porous sublayer. The actual selectivity is determined by the thin toplayer, whereas the porous sublayer serves as support. The techniques can be used, such as:

o   Dip coating

An asymmetric membrane (hollow fiber, flat sheets, often used in ultrafiltration) is immersed in the coating solution containing the polymer, prepolymer, or monomer, with concentration of solute low (less than 1%). When the asymmetric membrane is removed from the bath containing the coating material and the solvent, a thin layer of solution adheres to it.This film is then put in an oven where the solvent evaporates and where crosslinking also occurs. Such crosslinking leads to the thin layer becoming fixed to the porous sublayer.

o   Plasma polymerisation

The plasma being obtained by the ionisation of a gas by means of an electrical discharge at high frequencies up to 10 MHz. On entering the reactor, the gas is ionised and by ensuring that the reactants are supplied separately to the reactor, all kinds of radicals will be formed through collisions with the ionised gas which are capable of reacting with each other. The resulting product will precipitate when their molecular weight becomes too high. A very thin layer of thickness 50 nm can be obtained. Factors important are concentration of monomer in reactor, polymerisation time, vacuum pressure, gas flow, gas pressure, and frequency.

o   Interfacial polymerisation

A polymerisation reaction occurs between two very reactive monomers at the interface of two immicible solvents. The support layer is immersed in an aqueous solution containing a reactive monomer or a pre-polymer, frequently of the amine-type. The film is then immersed in a second bath containing a water-immisible solvent in which another reactive monomer, often an acid chloride, has been dissolved. These two reactive monomers (amine and acid chloride) react with each other to form a dense polymeric toplayer. Heat treatment is often appllied to complete the interfacial reaction and to crosslink the water-soluble monomer or pre-polymer. This method resulting an extremely thin film of thickness 50 nm.

o   In-situ polymerisation

o   Grafting

o   Spray coating

o   Spin coating

·         Phase inversion

Process whereby a polymer is transformed in a controlled manner from a liquid to a solid state, often initiated by the transition from one liquid state into two liquids (liquid-liquid demixing. One of the state will solidify so that a solid matrix is formed. The techniques such as:

o   Precipitation by a solvent evaporation

A polymer is dissolved in a solvent and the polymer solution is cast on a suitable support (e.g. a glass plate), which may be porous or non porous. The solvent is allowed to evaporate in an inert (e.g. nitrogen) atmosphere to exclude water vapour, aloowing a dense homogenous membrane to be obtained.

o   Precipitation from the vapour phase

A cast film, consisting osf a polymer and a solvent, is placed in a vapour phase consists of a nonsolvent saturated with the same solvent. The high solvent concentration in the vapour phase prevents the evaporation of solvent from the cast film. Membrane formation occurs because of the penetration (diffusion) of nonsolvent into the cast film. This leads to a porous membrane without toplayer.

o   Precipitation by controlled evaporation

The polymer is dissolved in a mixture of solvent and nonsolvent. Since the solvent is more volatile, the composition shits during evaporation to a higher nonsolvent and polymer content. This leads to the polymer precipitation leading to formation of a skinned membrane.

o   Thermal precipitation

A solution of polymer in a mixed or single solvent is cooled to enable phase separation to occur. Evaporation of the solvent often allows the formation of a skinned membrane.

o   Immersion precipitation

A polymer solution (polymer + solvent) is cast on a suitable support and immersed in a coagulation bath cantaining a nonsolvent. Precipitation occurs because of the exchange of solvent and nonsolvent. The membrane structure obtained results from a combination of mass transfer and phase separation. Immersion precipitation can prepare membranes in two configurations:

1)      Flat membranes

Flat membranes are used in plate-and-frame and spural-wound mhereas tubular membranes are used in hollow fiber, capillary, and tubular systems.

The polymer is dissolved in a solvent or solvent+additives mixture. The polymer solution is cast directly upon a supporting layer, for example a non-woven polyester, by means of a casting knife. The cast film is then immersed in a nonsolvent bath where exchange occurs between solvent and nonsolvent and eventually the polymer precipitates.

2)      Tubular membranes

A viscous polymer solution containing a polymer, solvent and sometimes additives is pump through a spinneret, the polymer solution being filtered before it enters the spinneret. After a short recidence time in the air, the fiber is immersed in a nonsolvent bath where coagulation occurs. The fiber is then collected upon a godet.