Introduction
1.4 General methods for membrane preparation
respectively [3]. Therefore, though ceramic membranes involve higher initial costs, their ability to provide higher flux and applicability to wide range of temperatures and chemical processing conditions could favor them to be the choice in contrary to the polymeric membranes. Though ceramic membrane possesses separation characteristics similar to polymeric membranes, due to higher cost, they are not yet widely applied in industrial scale applications. Under these circumstances, the development and usages of comparatively low cost ceramic membranes (200 - 400 $/m2) with longer life span (10 - 15 years) is anticipated to drive the economic competitiveness of ceramic membranes in the industry.
1.3.4 Polymer - ceramic composite membranes
Polymer-ceramic asymmetric composite membranes partly overcome some of the disadvantages of ceramic and polymer symmetric membranes. The asymmetric membranes possess wider pore size ranges achieved through the polymeric film layer deposited on the top of a ceramic support possessing higher chemical, mechanical and thermal stability. In addition, the asymmetric membranes use an optimal combination of both polymeric and ceramic membrane layers and hence the cost. Therefore, polymeric ceramic membranes perform widely better than the polymeric symmetric membranes with a marginally higher life span. These features accompanied by marginally higher cost (cost of ceramic support) are beneficial for applications where polymeric membranes are favored than the ceramic membranes. These applications include NF, RO, pervaporation, and gas separation.
polymer and polymer-ceramic asymmetric membranes. Polymeric membrane preparation methods are not discussed here as the present thesis does not deal with the subject matter.
1.4.1 Symmetric ceramic membranes
Symmetric ceramic membranes are fabricated using uni-axial and paste methods [6, 7]. Both these methods involve the preparation of an inorganic mixture using suitable pore forming organic and inorganic materials along with binder materials. The uni-axial method involves casting an inorganic mixture in a suitable disk or tubular shape and kept under very high pressure (30 - 50 MPa). Subsequently, the disk or tubular type mould is sintered to prepare the membrane. The paste method involves the preparation of a paste using the inorganic mixture using suitable solvent which is eventually casted into suitable shape and sintered at high temperature. The properties of the ceramic membrane are largely influenced by the composition of the raw materials, sintering temperature and process including the schedule of heating, sintering and cooling.
1.4.2 Asymmetric ceramic membranes
Asymmetric ceramic membranes are prepared by the deposition of thin film of inorganic materials over the porous symmetric ceramic membranes (support). Slip coating followed with casting, sol-gel and dip coating methods are usually deployed for the fabrication of asymmetric ceramic membranes whose properties are dependent upon both support as well as thin skin layer morphology [6, 7]. The slip coating-sintering process involves coating the support surface with a suspension of finer particles in a solution of a cellulosic polymer or poly vinyl alcohol that acts as a binder and viscosity enhancer to hold the particles in the suspension. Subsequently, the membrane is dried and sintered at high temperatures to obtain a
fine microporous surface layer. Usually, several slip-coated layers are applied in series with each layer formed from a suspension of progressively finer particles to yield an isotropic structure. Many commercial ceramic UF membranes are made using slip-coating technique to achieve skin layer pore diameters upto 100 - 200 oA. Membranes with even finer pore sizes are fabricated using the sol-gel technique. The sol-gel process involves the transition of the slip coating technique to the colloidal level. The colloidal or polymeric gel solutions of an inorganic hydroxide are initially prepared using controlled hydrolysis of metal salts or metal alkoxides. The particulate-sol method involves the dissolution of alkoxide dissolved in alcohol is followed with the hydrolysis by addition of excess water or acid. The precipitate thus obtained is maintained as a hot solution for extended time periods to yield a stable colloidal solution. The colloidal solution is then cooled and coated onto the microporous membrane support. Later, the membrane is carefully dried to avoid surface defects such as cracks and sintered at 500 - 800 oC. During dip coating process, the support is dipped in an inorganic suspension and the inorganic materials penetrate through the pores of the support and as well as deposit over the support surface. Subsequently, the membrane is dried and sintered at high temperature.
1.4.3 Polymer - ceramic composite membranes
Polymer-ceramic composite membranes are prepared by deposition of polymeric film over a porous ceramic support. Polymer-ceramic composite membranes characterized with superior combinations of structural integrity, fouling resistance, flux and selectivity have been reported for the UF [8-10] and pervaporation (PV) [11-15] applications. These membranes constitute a polymer skin layer consisting of polysulfone [9], styrene acrylonitrile [10], poly vinyl acetate (PVAc), poly vinyl pyrrolidone (PVP) [11], cellulose acetate (CA) [12], polydimethylsiloxane
(PDMS) [15, 16] and ceramic supports with nominal pore sizes of 0.2 – 2 µm made from kaolin [10], alumina [11] and zirconia [16]. There are several methods available for the preparation of polymer - ceramic composite membrane such as spray coating [10], grafting [11], spin coating [12], self assembly [14], dip coating [16] and vapor deposition [17]. The dip coating technique involves dipping the ceramic membrane support in the polymeric sol for a specified amount of time to yield the polymer-ceramic membrane. Spray coating, spin coating and self assembly are modified versions of dip-coating technique and are applied depending upon various issues such as the physio-chemical properties of the polymeric sol as well as the ceramic support. While spray coating involves spraying the polymeric sol onto the support, spin coating involves rotation of the support in a polymeric sol. Graft polymerization method involves three steps namely pretreatment, surface activation and grafting the polymer over the support. Therefore, though the method is efficient, it is marginally complicated when compared to other fabrication methods. The vapor deposition method involves deposition of the polymeric film by condensation of vapor on the membrane support. Among several fabrication methods outlined, dip coating method is simple, inexpensive and most desirable choice for industrial applications. This is due to the fact that dip coating techniques involves simple fabrication set up which can be used along with an optimal combination of dip coating parameters such as concentration of the polymer, support morphology and time of dip coating to yield the polymer-ceramic composite membrane within a very short span of time.