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Characterization of heterogeneous anion-exchange membrane
Journal of Membrane Science 187 (2001) 39–46
Characterization of heterogeneous anion-exchange membrane Punita V. Vyas, B.G. Shah, G.S. Trivedi, P. Ray, S.K. Adhikary∗ , R. Rangarajan Central Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, Gujarat State, India Received 27 June 2000; received in revised form 22 September 2000; accepted 6 October 2000
Abstract This paper reports the studies on the properties of heterogeneous anion-exchange membranes prepared from polyvinyl chloride (PVC) as binder and anion-exchange resin powder. The effect of variation of ion-exchange resin particle size as well as the resin–binder ratio on different mechanical, electrochemical and morphological properties of the membranes have also been studied. It has been found that by using resins of suitable particle size and loading, it is possible to achieve heterogeneous membranes which are comparable with interpolymer membranes in performance. Moreover, heterogeneous membranes are found to have more dimensional stability than interpolymer membranes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Anion-exchange membrane; PVC; Anion-exchange resin; Electrochemical properties; Mechanical strength
1. Introduction Ion-exchange membranes, both the homogeneous and heterogeneous, being unique in their nature supercedes each other in one way or another. Homogeneous membranes having good electrochemical properties lack in their mechanical strength, whereas heterogeneous membranes having very good mechanical strength are comparatively poor in their electrochemical performances. Early references [1] on heterogeneous ion-exchange membranes reveal that it may be made by mechanical incorporation of powdered ion-exchange resin into sheets of rubber, PVC, acrylonitrile copolymers or some other extrudable or mouldable matrix. Such membranes can be prepared [2] either by (i) calendering ion-exchange particles into an inert plastic film or (ii) dry moulding of inert film forming polymers and ion-exchange particles and then milling the ∗ Corresponding author. +91-278-567-760; fax: +91-278-566-970. E-mail address: [email protected] (S.K. Adhikary).
mould stock or (iii) resin particles can be dispersed in a solution containing a film forming binder and then the solvent is evaporated to give ion-exchange membrane. Such heterogeneous membranes may also be reinforced with a chemically resistant fabric [3]. Interpolymer ion-exchange membranes [4–6] possessing an excellent combination of both electrochemical and mechanical properties have been successfully developed by this Institute. But at the same time, successful trials have also been made for the synthesis of heterogeneous ion-exchange membranes. The present study relates to the studies on the properties of heterogeneous anion-exchange membrane based on PVC as a binder in which anion-exchange resin particles are dispersed. Although references [7–9] have been found for the synthesis of the heterogeneous anion-exchange membranes based on PVC or other binders, the speciality of this investigation lies in the elaborate study of the effect of variation of ion-exchange resin particle size and the loading of the resin on different mechanical, electrochemical and morphological properties of the membranes.
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 6 1 3 - X
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P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
2. Experimental 2.1. Preparation of anion-exchange membranes Polyvinyl chloride (PVC), flexible lamination film grade, supplied by IPCL, India, grade 67 GEF 092, k value = 67, tetrahydrofuran(THF), LR grade, as solvent with a refractive index of 1.407–1.409 and a density of 0.886–0.888 g/cm3 at 25◦ C and anion-exchange resin (Indion FFIP), a chloromethylated and aminated polystyrene (with 8% cross-link density), supplied by Ion-exchange (India) Ltd., with an exchange capacity of 3.4 meq/g dry resin were used to prepare the membranes. Anion-exchange resin particles were dried in an oven at 60◦ C for 24 h, powdered in a ball mill and sieved to the desired mesh size. Finely powdered ion-exchange resin was then dispersed in solution of polyvinyl chloride (PVC) in THF in which total solid:THF ratio was 1:10 (w/v) and then membranes were prepared using spray coating technique. The membranes were dried at an ambient temperature (∼35◦ C) for 30 min and the almost dried membranes were immersed in water. Different mesh size and loading of resin were used to prepare membranes for their comparative studies. The membranes thus prepared were conditioned by equilibrating in (∼1N) NaOH solution and subsequently in NaCl solution. 2.2. Evaluation of properties of anion-exchange membranes Mechanical properties like bursting strength, physico-chemical properties, namely (i) dimensional
changes in different ionic forms, (ii) dimensional changes in different concentrations of equilibrating solutions of sodium chloride, (iii) ion-exchange capacity and moisture content, and different electrochemical properties like areal electrical resistance and transport number of the membranes were measured with the help of procedures described earlier [6,10].
3. Results and discussion Anion-exchange membranes thus obtained are heterogeneous in nature with dispersed ion-exchange resin particles in the base polyvinyl chloride (PVC) matrix. The particle size distribution of anion-exchange resin powder of different mesh, as shown in Fig. 1, was studied with the help of Mastersiser 2000 of Malvern, UK using water as the dispersing agent. Fig. 1 reveals that the average particle size of 90% of the resins having mesh sizes of −100 + 200 BSS, −200 + 300 BSS and −300 + 400 BSS are 80, 70, and 39 m, respectively. Ion-exchange resin particles used are cross-linked polystyrene (PS) having ion-exchangeable functional groups. It is observed that with increase in resin loading the membranes become more and more brittle and at the same time it is also observed that the finer the resin particles are, the more flexible is the membrane. Hence with resin particles of −300 + 400 mesh (39 m), it is possible to obtain flexible membranes upto 60% resin loading whereas the membranes become brittle even with 40% loading for −100 + 200 mesh sized (80 m) resin particles. With increase in
Fig. 1. Particle size vs. volume; (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
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Fig. 2. Bursting strength vs. resin loading in membranes having different mesh size of resin particles; (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
resin loading, after a certain point phase inversion takes place, and cross-linked PS particles tend to form the continuous phase and PVC as the discrete phase. PS being more brittle in nature compared to plasticized PVC (plasticized with 3% di-butyl phthalate), fails to act as an impact modifier and crack propagation becomes facile, resulting in a brittle membrane. At any particular blend ratio the finer the resin particles, the more homogeneous the blend is, resulting in a more flexible membrane. The bursting strength (Fig. 2) of such blend system decreases with increase in resin loading. This may be due to the fact that as the resin loading increases, the cross-linked PS particles tend to form a coninuous phase and having a very less impact strength fails to dissipate the absorbed energy. Hence, crack propagation becomes faster. It is also observed that (Fig. 2) at any particular blend ratio membranes having higher particle size of resin exhibit higher bursting strength. The morphological character of different heterogeneous membranes was studied by optical microscope (Olympus IX 70) at a magnification of 40×. The effects of variation of resin–binder ratio at a mesh size −100 + 200 BSS are shown in Fig. 3A (i)–(iii). As the resin content increases from 40 to 70% it can be seen from micrograph (i) to (iii), that a clear cut phase inversion takes place. Micrograph (i) exhibits PVC as a continuous phase having dispersed resin particles in it,
whereas in micrograph (iii), resin phase is the continuous one. The phase morphology is found to be more homogeneous with respect to the distribution of resin particles in micrograph (iii) compared to the others. Ion-exchange resin being insoluble and infusible are incapable to form a film. PVC, being a film forming material, is used as a binder to impart the mechanical strength to the membrane. Therefore, when PVC becomes a minor component, invariably the membrane loses its mechanical strength and exhibits inferior mechanical properties (Fig. 2). The effect of variation of mesh size of the resin particles, at a particular resin–binder composition, on the phase morphology of the membranes is shown in Fig. 3B (i)–(iii). It is seen that the finer the resin particles are, the more homogeneous and uniform is the phase morphology. Keeping in view the practical applications, the dimensional stability in different ionic forms is the most desirable criteria for any commercially successful ion-exchange membrane. For these heterogeneous anion-exchange membranes synthesized in our laboratory, it is observed that there is no noticeable change in the area of the membrane from one ionic form to an2− − other, i.e. OH → Cl− , OH− → NO− 3 , OH → SO4 3− − and OH → PO4 . It is also observed that there is no appreciable change in the dimension of the membrane in different ion concentrations of NaCl solution
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Fig. 3. (A) Optical micrographs of heterogeneous membranes, magnification 40×. Effect of variation of resin–binder ratio. (i) 40:60; (ii) 60:40; and (iii) 70:30; at a mesh size of −100 + 200 BSS. (B) Optical micrographs of heterogeneous membranes, magnification 40×. Effect of variation of mesh size at 60:40 resin, binder composition. (i) −100 + 200 BSS; (ii) −200 + 300 BSS; and (iii) −300 + 400 BSS.
P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
Fig. 3. (Continued).
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Fig. 4. Areal resistance vs. resin loading in membranes having different mesh size of resin particles; thickness of the membranes: 0.2 mm. (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
(say 2000, 4000, 8000 and 12,000 ppm) whereas the interpolymer anion-exchange membrane as reported earlier [6] exhibits appreciable expansion/contraction in different ionic forms of the membrane and also in different concentration of NaCl solution. It is pertinent at this juncture to discuss the differences that arises in the morphology of interpolymer type and heterogeneous type of membranes. In the preparation of interpolymer membrane, linear polyethylene (PE), is used as a binder. It is made into an organosol with suitable solvating monomers like styrene-divinyl benzene (DVB). This blend, on polymerization of monomers under a free radical mechanism yields a chemical polyblend of two interpenetrating networks of linear and cross-linked polymer molecules [4,6]. In addition, due to grafting there may be an inter cross-linking between PE and PS phases. Such a chemical polyblend behaves like a homogeneous type having less probability of micro voids. For lack of such available space, when there is solvation of ionic groups, there is an expansion of matrix with attendant dimensional changes. In the case of heterogeneous membranes, on the other hand, the loss of solvent due to evaporation introduces microvoids between the resin and PVC regions, sufficient to accommodate
solvent molecules solvating the ionic species in the resin. Therefore, as a result of solvation, dimensional changes do not manifest. Apart from dimensional stability the other two most important characteristics of ion-exchange membranes are its ion-exchange capacity and electrical resistance. The membranes should have reasonable ion-exchange capacity with very low electrical resistance. As expected, for membranes of same thickness (0.2 mm in this case) the areal resistance decreases (Fig. 4) and capacity increases (Fig. 5) with increase in resin loading. The particle size of the resin also affects the ion-exchange capacity and areal resistance of the membrane (Figs. 4 and 5). At a definite resin loading and thickness of the membrane, the electrical resistance decreases and ion-exchange capacity increases with decrease in particle size (or increase in mesh size). Decrease in particle size results in the higher surface area (measured by ASAP-2010, Micromerities, Norcross, USA, at liquid nitrogen temperature) of the resin particles. Surface area increases from 2.6261 to 4.5168 m2 /g. With increase in surface area, the number of functional groups which may actively participate in the transport of counter ions through the
P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
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Fig. 5. Exchange capacity vs. resin loading in membranes; thickness of the membranes: 0.2 mm. (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
membrane increases which results in lower resistance and higher ion-exchange capacity of the membranes. The transport numbers of these heterogeneous membranes remain unaffected with the variation of resin content of the membrane. From the above discussion it is very clear that properties of heterogeneous anion-exchange membranes depend upon the relative proportion of the binder and the ion-exchange resin. Membranes having higher binder content possess good mechanical strength but they lack in their electrochemical properties whereas membranes having higher resin content possess good electrochemical properties but their mechanical properties are comparatively poor. Both these mechanical and electrochemical properties also depend on the particle size of the resin. So to achieve a better heterogeneous membrane, a compromise between the electrochemical and mechanical properties is needed, i.e. an optimization of the resin–binder ratio and particle size is essential. From a thorough analysis of the data it is concluded that the membrane with thickness 0.2 mm prepared from resin having average particle size (90% of the resin) of 39 m (−300 + 400 BSS mesh) with 60% resin loading has bursting strength
of 1.9 kg/cm2 , areal resistance 8–9 cm2 , capacity 2.14 meq/g and transport no. 0.90.
4. Conclusions • Heterogeneous anion-exchange membranes having better dimensional stabilities and almost identical electrochemical properties and mechanical strengths as those of interpolymer anion-exchange membranes prepared early in this Institute, can be prepared from PVC and anion-exchange resin by selecting suitable particle size and loading of the resin. • Unlike interpolymer membranes, heterogeneous anion-exchange membranes prepared from PVC and anion-exchange resin, do not require multiple stages of preparation. • The mechanical strength of the membrane can be improved by using suitable reinforcing material. In such case, resin loading is to be adjusted to have acceptable electrical resistance. • The heterogeneous membrane can be a good substitute of interpolymer membranes in ED applications.
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P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
Acknowledgements Authors are very much thankful to the Ministry of Environment & Forests, Govt. of India, for funding a project to carry out this investigation. Authors are also thankful to Mr. Vijayraghavan for typing this manuscript. References [1] G.W. Bodamer (Rohm and Hass), US Patents 2,681,319 (1954), 2,681,320 (1954), 2,737,486 (1956). [2] W.S. Winston Ho, K.K Sarkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, New York, 1992. [3] H.F. Mark, N.G. Gaylord (Eds.), Encyclopedia of Polymer Science and Technology, Vol. 8, Wiley, New York, 1968, p. 633. [4] K.P. Govindan, P.K. Narayanan, Indian Patent 124,573 (1969). [5] P.K. Narayanan, S.K. Adhikary, W.P. Harkare, K.P. Govindan, Indian Patent 160,880 (1987).
[6] S.K. Adhikary, N.J. Dave, P.K. Narayanan, W.P. Harkare, B.S. Joshi, K.P. Govindan, Studies on interpolymer membranes. Part III. Cation excahange membranes, React. Polym. 1 (1983) 197. [7] G. Radulescu, A. Sipos, S. Szabo, V. Sipos, M.I. Moise, P. Szentgyorgyi, Production of heterogeneous anion-exchange membranes. Rev. Chim. (Bucharest) 41 (5/6) (1990) 511, Chem. Abstr. 114 (23/24) (1991) 23026. [8] E. Zh. Menligaziev, E.E. Ergozhin, M.P. Blimova, K. Kh Tastanov, Anion-exchange membranes produced from epoxidized aromatic diamines, Izv. Akad. Nauk Kaz. SSR, Sez. Khim. 1 (1988) 71, Chem. Abstr. 108 (17/18) (1988) 151429. [9] T.-S. Hwang, J.R. Choi, Preparation and properties of heterogeneous anion-exchange membrane from polyethylene matrix with 4-vinylpyridine-divinyl benzene resin, Han’guk Chaelyo Hakholchi 8 (11) (1998) 1061, Chem. Abstr. 130 (12) (1999) 154372. [10] G. Ramachandraiah, P. Ray, Electroassisted transport phenomenon of strong and weak electrolytes across ion-exchange membranes: chronopotentiometric study on deactivation of anion-exchange membranes by higher homologous mono-carboxylates, J. Phys. Chem. 101 (1997) 7892.
Characterization of heterogeneous anion-exchange membrane Punita V. Vyas, B.G. Shah, G.S. Trivedi, P. Ray, S.K. Adhikary∗ , R. Rangarajan Central Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, Gujarat State, India Received 27 June 2000; received in revised form 22 September 2000; accepted 6 October 2000
Abstract This paper reports the studies on the properties of heterogeneous anion-exchange membranes prepared from polyvinyl chloride (PVC) as binder and anion-exchange resin powder. The effect of variation of ion-exchange resin particle size as well as the resin–binder ratio on different mechanical, electrochemical and morphological properties of the membranes have also been studied. It has been found that by using resins of suitable particle size and loading, it is possible to achieve heterogeneous membranes which are comparable with interpolymer membranes in performance. Moreover, heterogeneous membranes are found to have more dimensional stability than interpolymer membranes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Anion-exchange membrane; PVC; Anion-exchange resin; Electrochemical properties; Mechanical strength
1. Introduction Ion-exchange membranes, both the homogeneous and heterogeneous, being unique in their nature supercedes each other in one way or another. Homogeneous membranes having good electrochemical properties lack in their mechanical strength, whereas heterogeneous membranes having very good mechanical strength are comparatively poor in their electrochemical performances. Early references [1] on heterogeneous ion-exchange membranes reveal that it may be made by mechanical incorporation of powdered ion-exchange resin into sheets of rubber, PVC, acrylonitrile copolymers or some other extrudable or mouldable matrix. Such membranes can be prepared [2] either by (i) calendering ion-exchange particles into an inert plastic film or (ii) dry moulding of inert film forming polymers and ion-exchange particles and then milling the ∗ Corresponding author. +91-278-567-760; fax: +91-278-566-970. E-mail address: [email protected] (S.K. Adhikary).
mould stock or (iii) resin particles can be dispersed in a solution containing a film forming binder and then the solvent is evaporated to give ion-exchange membrane. Such heterogeneous membranes may also be reinforced with a chemically resistant fabric [3]. Interpolymer ion-exchange membranes [4–6] possessing an excellent combination of both electrochemical and mechanical properties have been successfully developed by this Institute. But at the same time, successful trials have also been made for the synthesis of heterogeneous ion-exchange membranes. The present study relates to the studies on the properties of heterogeneous anion-exchange membrane based on PVC as a binder in which anion-exchange resin particles are dispersed. Although references [7–9] have been found for the synthesis of the heterogeneous anion-exchange membranes based on PVC or other binders, the speciality of this investigation lies in the elaborate study of the effect of variation of ion-exchange resin particle size and the loading of the resin on different mechanical, electrochemical and morphological properties of the membranes.
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 6 1 3 - X
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P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
2. Experimental 2.1. Preparation of anion-exchange membranes Polyvinyl chloride (PVC), flexible lamination film grade, supplied by IPCL, India, grade 67 GEF 092, k value = 67, tetrahydrofuran(THF), LR grade, as solvent with a refractive index of 1.407–1.409 and a density of 0.886–0.888 g/cm3 at 25◦ C and anion-exchange resin (Indion FFIP), a chloromethylated and aminated polystyrene (with 8% cross-link density), supplied by Ion-exchange (India) Ltd., with an exchange capacity of 3.4 meq/g dry resin were used to prepare the membranes. Anion-exchange resin particles were dried in an oven at 60◦ C for 24 h, powdered in a ball mill and sieved to the desired mesh size. Finely powdered ion-exchange resin was then dispersed in solution of polyvinyl chloride (PVC) in THF in which total solid:THF ratio was 1:10 (w/v) and then membranes were prepared using spray coating technique. The membranes were dried at an ambient temperature (∼35◦ C) for 30 min and the almost dried membranes were immersed in water. Different mesh size and loading of resin were used to prepare membranes for their comparative studies. The membranes thus prepared were conditioned by equilibrating in (∼1N) NaOH solution and subsequently in NaCl solution. 2.2. Evaluation of properties of anion-exchange membranes Mechanical properties like bursting strength, physico-chemical properties, namely (i) dimensional
changes in different ionic forms, (ii) dimensional changes in different concentrations of equilibrating solutions of sodium chloride, (iii) ion-exchange capacity and moisture content, and different electrochemical properties like areal electrical resistance and transport number of the membranes were measured with the help of procedures described earlier [6,10].
3. Results and discussion Anion-exchange membranes thus obtained are heterogeneous in nature with dispersed ion-exchange resin particles in the base polyvinyl chloride (PVC) matrix. The particle size distribution of anion-exchange resin powder of different mesh, as shown in Fig. 1, was studied with the help of Mastersiser 2000 of Malvern, UK using water as the dispersing agent. Fig. 1 reveals that the average particle size of 90% of the resins having mesh sizes of −100 + 200 BSS, −200 + 300 BSS and −300 + 400 BSS are 80, 70, and 39 m, respectively. Ion-exchange resin particles used are cross-linked polystyrene (PS) having ion-exchangeable functional groups. It is observed that with increase in resin loading the membranes become more and more brittle and at the same time it is also observed that the finer the resin particles are, the more flexible is the membrane. Hence with resin particles of −300 + 400 mesh (39 m), it is possible to obtain flexible membranes upto 60% resin loading whereas the membranes become brittle even with 40% loading for −100 + 200 mesh sized (80 m) resin particles. With increase in
Fig. 1. Particle size vs. volume; (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
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Fig. 2. Bursting strength vs. resin loading in membranes having different mesh size of resin particles; (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
resin loading, after a certain point phase inversion takes place, and cross-linked PS particles tend to form the continuous phase and PVC as the discrete phase. PS being more brittle in nature compared to plasticized PVC (plasticized with 3% di-butyl phthalate), fails to act as an impact modifier and crack propagation becomes facile, resulting in a brittle membrane. At any particular blend ratio the finer the resin particles, the more homogeneous the blend is, resulting in a more flexible membrane. The bursting strength (Fig. 2) of such blend system decreases with increase in resin loading. This may be due to the fact that as the resin loading increases, the cross-linked PS particles tend to form a coninuous phase and having a very less impact strength fails to dissipate the absorbed energy. Hence, crack propagation becomes faster. It is also observed that (Fig. 2) at any particular blend ratio membranes having higher particle size of resin exhibit higher bursting strength. The morphological character of different heterogeneous membranes was studied by optical microscope (Olympus IX 70) at a magnification of 40×. The effects of variation of resin–binder ratio at a mesh size −100 + 200 BSS are shown in Fig. 3A (i)–(iii). As the resin content increases from 40 to 70% it can be seen from micrograph (i) to (iii), that a clear cut phase inversion takes place. Micrograph (i) exhibits PVC as a continuous phase having dispersed resin particles in it,
whereas in micrograph (iii), resin phase is the continuous one. The phase morphology is found to be more homogeneous with respect to the distribution of resin particles in micrograph (iii) compared to the others. Ion-exchange resin being insoluble and infusible are incapable to form a film. PVC, being a film forming material, is used as a binder to impart the mechanical strength to the membrane. Therefore, when PVC becomes a minor component, invariably the membrane loses its mechanical strength and exhibits inferior mechanical properties (Fig. 2). The effect of variation of mesh size of the resin particles, at a particular resin–binder composition, on the phase morphology of the membranes is shown in Fig. 3B (i)–(iii). It is seen that the finer the resin particles are, the more homogeneous and uniform is the phase morphology. Keeping in view the practical applications, the dimensional stability in different ionic forms is the most desirable criteria for any commercially successful ion-exchange membrane. For these heterogeneous anion-exchange membranes synthesized in our laboratory, it is observed that there is no noticeable change in the area of the membrane from one ionic form to an2− − other, i.e. OH → Cl− , OH− → NO− 3 , OH → SO4 3− − and OH → PO4 . It is also observed that there is no appreciable change in the dimension of the membrane in different ion concentrations of NaCl solution
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Fig. 3. (A) Optical micrographs of heterogeneous membranes, magnification 40×. Effect of variation of resin–binder ratio. (i) 40:60; (ii) 60:40; and (iii) 70:30; at a mesh size of −100 + 200 BSS. (B) Optical micrographs of heterogeneous membranes, magnification 40×. Effect of variation of mesh size at 60:40 resin, binder composition. (i) −100 + 200 BSS; (ii) −200 + 300 BSS; and (iii) −300 + 400 BSS.
P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
Fig. 3. (Continued).
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Fig. 4. Areal resistance vs. resin loading in membranes having different mesh size of resin particles; thickness of the membranes: 0.2 mm. (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
(say 2000, 4000, 8000 and 12,000 ppm) whereas the interpolymer anion-exchange membrane as reported earlier [6] exhibits appreciable expansion/contraction in different ionic forms of the membrane and also in different concentration of NaCl solution. It is pertinent at this juncture to discuss the differences that arises in the morphology of interpolymer type and heterogeneous type of membranes. In the preparation of interpolymer membrane, linear polyethylene (PE), is used as a binder. It is made into an organosol with suitable solvating monomers like styrene-divinyl benzene (DVB). This blend, on polymerization of monomers under a free radical mechanism yields a chemical polyblend of two interpenetrating networks of linear and cross-linked polymer molecules [4,6]. In addition, due to grafting there may be an inter cross-linking between PE and PS phases. Such a chemical polyblend behaves like a homogeneous type having less probability of micro voids. For lack of such available space, when there is solvation of ionic groups, there is an expansion of matrix with attendant dimensional changes. In the case of heterogeneous membranes, on the other hand, the loss of solvent due to evaporation introduces microvoids between the resin and PVC regions, sufficient to accommodate
solvent molecules solvating the ionic species in the resin. Therefore, as a result of solvation, dimensional changes do not manifest. Apart from dimensional stability the other two most important characteristics of ion-exchange membranes are its ion-exchange capacity and electrical resistance. The membranes should have reasonable ion-exchange capacity with very low electrical resistance. As expected, for membranes of same thickness (0.2 mm in this case) the areal resistance decreases (Fig. 4) and capacity increases (Fig. 5) with increase in resin loading. The particle size of the resin also affects the ion-exchange capacity and areal resistance of the membrane (Figs. 4 and 5). At a definite resin loading and thickness of the membrane, the electrical resistance decreases and ion-exchange capacity increases with decrease in particle size (or increase in mesh size). Decrease in particle size results in the higher surface area (measured by ASAP-2010, Micromerities, Norcross, USA, at liquid nitrogen temperature) of the resin particles. Surface area increases from 2.6261 to 4.5168 m2 /g. With increase in surface area, the number of functional groups which may actively participate in the transport of counter ions through the
P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
45
Fig. 5. Exchange capacity vs. resin loading in membranes; thickness of the membranes: 0.2 mm. (1) −100 + 200 BSS; (2) −200 + 300 BSS; and (3) −300 + 400 BSS.
membrane increases which results in lower resistance and higher ion-exchange capacity of the membranes. The transport numbers of these heterogeneous membranes remain unaffected with the variation of resin content of the membrane. From the above discussion it is very clear that properties of heterogeneous anion-exchange membranes depend upon the relative proportion of the binder and the ion-exchange resin. Membranes having higher binder content possess good mechanical strength but they lack in their electrochemical properties whereas membranes having higher resin content possess good electrochemical properties but their mechanical properties are comparatively poor. Both these mechanical and electrochemical properties also depend on the particle size of the resin. So to achieve a better heterogeneous membrane, a compromise between the electrochemical and mechanical properties is needed, i.e. an optimization of the resin–binder ratio and particle size is essential. From a thorough analysis of the data it is concluded that the membrane with thickness 0.2 mm prepared from resin having average particle size (90% of the resin) of 39 m (−300 + 400 BSS mesh) with 60% resin loading has bursting strength
of 1.9 kg/cm2 , areal resistance 8–9 cm2 , capacity 2.14 meq/g and transport no. 0.90.
4. Conclusions • Heterogeneous anion-exchange membranes having better dimensional stabilities and almost identical electrochemical properties and mechanical strengths as those of interpolymer anion-exchange membranes prepared early in this Institute, can be prepared from PVC and anion-exchange resin by selecting suitable particle size and loading of the resin. • Unlike interpolymer membranes, heterogeneous anion-exchange membranes prepared from PVC and anion-exchange resin, do not require multiple stages of preparation. • The mechanical strength of the membrane can be improved by using suitable reinforcing material. In such case, resin loading is to be adjusted to have acceptable electrical resistance. • The heterogeneous membrane can be a good substitute of interpolymer membranes in ED applications.
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P.V. Vyas et al. / Journal of Membrane Science 187 (2001) 39–46
Acknowledgements Authors are very much thankful to the Ministry of Environment & Forests, Govt. of India, for funding a project to carry out this investigation. Authors are also thankful to Mr. Vijayraghavan for typing this manuscript. References [1] G.W. Bodamer (Rohm and Hass), US Patents 2,681,319 (1954), 2,681,320 (1954), 2,737,486 (1956). [2] W.S. Winston Ho, K.K Sarkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, New York, 1992. [3] H.F. Mark, N.G. Gaylord (Eds.), Encyclopedia of Polymer Science and Technology, Vol. 8, Wiley, New York, 1968, p. 633. [4] K.P. Govindan, P.K. Narayanan, Indian Patent 124,573 (1969). [5] P.K. Narayanan, S.K. Adhikary, W.P. Harkare, K.P. Govindan, Indian Patent 160,880 (1987).
[6] S.K. Adhikary, N.J. Dave, P.K. Narayanan, W.P. Harkare, B.S. Joshi, K.P. Govindan, Studies on interpolymer membranes. Part III. Cation excahange membranes, React. Polym. 1 (1983) 197. [7] G. Radulescu, A. Sipos, S. Szabo, V. Sipos, M.I. Moise, P. Szentgyorgyi, Production of heterogeneous anion-exchange membranes. Rev. Chim. (Bucharest) 41 (5/6) (1990) 511, Chem. Abstr. 114 (23/24) (1991) 23026. [8] E. Zh. Menligaziev, E.E. Ergozhin, M.P. Blimova, K. Kh Tastanov, Anion-exchange membranes produced from epoxidized aromatic diamines, Izv. Akad. Nauk Kaz. SSR, Sez. Khim. 1 (1988) 71, Chem. Abstr. 108 (17/18) (1988) 151429. [9] T.-S. Hwang, J.R. Choi, Preparation and properties of heterogeneous anion-exchange membrane from polyethylene matrix with 4-vinylpyridine-divinyl benzene resin, Han’guk Chaelyo Hakholchi 8 (11) (1998) 1061, Chem. Abstr. 130 (12) (1999) 154372. [10] G. Ramachandraiah, P. Ray, Electroassisted transport phenomenon of strong and weak electrolytes across ion-exchange membranes: chronopotentiometric study on deactivation of anion-exchange membranes by higher homologous mono-carboxylates, J. Phys. Chem. 101 (1997) 7892.