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Hydrophilization of polyethylene membranes
Journal of Membrane Science, 50 (1990) 97-100 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
97
Short Communication
HYDROPHILIZATION
OF POLYETHYLENE
MEMBRANES
A. DIMOV and M.A. ISLAM* Higher Institute (Received
of Chemical Technology,
April 12,1989;
8010 Bow-gas (Bulgaria)
accepted in revised form October 20,1989)
Summary Polyethylene microfiltration membranes were hydrophilized by treating them with ternary mixtures of ethanol-water-inorganic acids. It was found that the threshold pressure necessary to initiate water permeation through the polyethylene membranes depends on the composition of hydrophilizing mixture. The membranes hydrophilized by the ternary solutions showed much better initial flux and lower flux decline rate as compared with those hydrophilized with ethanol alone. It was found that the hydrophilizing efficiency of the inorganic acid in the mixture decreases in the order HNO, > H&30, > H,PO,. However, the flux retention efficiency decreases in the reverse sequence.
Hydrophilization of polyolefin membranes is mostly performed by chemical modification. Well known modification methods are: treatment of the polyolefin membranes with sulphonating agents [ 1 ] and ozone [ 21, or grafting of hydrophilic monomers, such as acrylonitrile [3], acrylic acid [4] and others, onto the polymer macromolecules, so building up the membrane structure. The disadvantage of chemical modification is that, in most cases, it is accompanied by a process of destruction or a change in the porous structure of the membrane. Another kind of hydrophilization is a temporary one, which is achieved by wetting the polyolefin membranes with an organic water-soluble solvent [ 5,6]. After such physical treatment, capillary repulsive forces of the hydrophobic membranes cease to counteract the water flux. The basic disadvantage of this hydrophilization method, in which the chemical structure of the membrane does not change, is that the water flux decreases with time, an effect which is due to gradual leaching of the hydrophilizing agent by the water permeating through the membrane. In the present work, we have attempted to hydrophilize polyethylene membranes by treating them with an alcohol-acid mixture with a view to improve their permeability and working life. Membranes were prepared from a mixture of high-density polyethylene, sil*To whom correspondence
0376-7388/90/$03.50
should be addressed.
0 1990 Elsevier Science Publishers
B.V.
98
icon dioxide and oil. A film with a thickness of 0.4 mm was prepared from the molten polymer composition by pressing. Oil was extracted from the film by treating it with tetrachloroethylene. Membranes with 45% porosity and average pore radius 0.12 pm were formed from the film under the action of thermomechanical deformation. Water flux was determined with a laboratory cell of membrane area 12.56 cm’. Ethanol and a mixture of ethanol and inorganic acids were used as hydrophilizing agents. The membrane samples were immersed in hydrophilizing mixture for 5 min at a temperature of 20-40” C. Polyethylene membranes with average pore radius 0.12 pm do not show any detectable water flux at a pressure of 0.5 MPa. To overcome the capillary repulsive forces towards some non-wetting fluids, a corresponding superatmospheric pressure needs to be applied. Theoretically, according to Cantor’s relationship [ 71, hydrophobic polyethylene membranes with pore radius 0.12 ym are water permeable at a pressure of ca. 1.2 MPa. When the membranes are treated with alcohol, capillary repulsive forces are reduced significantly, and the experimentally determined water flux at a pressure of 0.1 MPa is 0.57 m”_m-2_hr-‘. The water flux as a function of the operating pressure is represented in Fig. 1. The measurements are performed at pressures up to 0.2 MPa in order to avoid membrane compaction, which may cause deviation of the relationship from Darcy’s law. The values of the pressure necessary to initiate water permeation depend on the composition of the hydrophilizing mixture. Some authors [ 81 observe the deviation of the curve from linearity as it approaches the origin. The above mentioned relationship remained linear in our experiments. It is obvious from the figure that, with an increase in the degree of hydrophilization, the curves may be described more accurately by Darcy’s law. At a constant pressure, the highest flux is observed for the membranes treated with a hydrophilizing mixture containing nitric acid.
12
_
0.4 _
Fig. 1. Water flux F as a function of the operating pressure P through membranes treated with different hydrophilizing mixtures: (1) 40 mol% ethanol; (2)) (3) and (4) ternary mixtures of 40 mol% ethanol;
(411.
53.8 mol% water and 6.2 mol% inorganic
acid
[H,PO,
(2), H,SO,
(3), HNOCI
99
06-
Fig. 2. Initial water flux F as a function of the composition of the hydrophilizing mixture; n is the molar fraction of ethanol in the alcohol-water mixture (1) and in the mixtures alcohol-wateinorganic acid [ H,PO, (2 ) , H,SO, (3 ) , HNO, (4) 1.The concentration of acid in all the compositions is 6.2 mol%. Operating pressure 0.1 MPa.
Figure 2 represents the relationship between the initial water flux through the polyethylene membrane and the composition of the hydrophilizing agent, consisting of either a binary ethanol-water mixture or a ternary mixture of water-ethanol-acid. In the ternary mixture, the quantity of the inorganic acid is kept constant (6.2 mol% ) with respect to the other two components (water and ethanol). A low temperature and a low concentration of acid during the treatment ensure the absence of chemical reaction between the membrane material and the hydrophilizing mixture. The results obtained may be explained by the greater affinity of esters than of the corresponding alcohol towards the membrane material. With an increase in the quantity of alcohol, the affinity of the mixture towards the membrane increases. Up to an alcohol concentration of 60 mol% in the ternary mixture, the water flux through the membrane increases quite rapidly, and the incremental rate then slows down. This phenomenon may be explained by the assumption that, beyond the above-mentioned alcohol concentration, a gradual saturation of the hydrophilizing capacity of the mixture takes place and the flux tends to a limiting value. On the other hand, the hydrophobicity follows the order: (CzH5)3P04> ( C,H,)zSO,> C2H5N03, owing to the decreasing contribution of organic ethyl groups in the esters; as a result, permeability increase in the same sequence. The more difficult removal of the hydrophilizing esters than of ethanol is shown in Fig. 3, which represents the water flux as a function of the duration of continuous flow of water through the polyethylene membrane treated with different hydrophilizing mixtures. After an experiment lasting 8 hr under the conditions described at the beginning of this paper, the water flux of membranes treated with mixtures containing nitric acid, sulphuric acid and phos-
Fig. 3. Water flux F as a function
of the duration
r of continuous
flow through the membranes
treated with different hydrophilizingcompositions: 100 mol% ethanol (1); 50 mol% ethanol, 43.8 mol% water, 6.2 mol% H,PO, (2); 40 mol% ethanol, 53.8 mol% water and 6.2 mol% H&SO4 (3) or HN03 (4). Operating pressure 0.15 MPa at entrance and 0.05 MPa at exit.
phoric acid was reduced by 8.7,6.7 and 5.4%, respectively. The temporary hydrophilicity achieved by treatment with the ethanol-water mixture loses 28.1% of its efficiency over the same operating period. It may be concluded that the water flux and the duration of the effective action of the hydrophilized polyethylene membranes may be controlled by changing the composition of the hydrophilizing mixtures.
References K. Matsuda, M. Kohno and Y. Doi, Hydrophilic sulphonated polyolefin porous membrane and process for preparing the same, U.S. Patent 4,409,339,1983. I. Hajima, H. Akira and Y. Toshio, Hydrophilization of porous hydrophilic membranes, Jpn. Patent 6,219,208, 1987. H. Saburo, H. Kumo and K. Yoshisuke, Patent 61,125,409, 1986.
Hydrophilic
polyolefin
hollow fibre membranes,
Jpn.
N. Kan, K. Koichi, M. Kenji, K. Mosato and T. Joichi, Hydrophilic polyethylene microporous membranes, Jpn. Patent 61,106,640, 1986. T.D. Brock, Membrane Filtration, Mir, Moscow, 1987. Mitsubishi Rayon Co. Ltd., Hydrophilic membranes for body fluid filtration, Jpn. Patent 5,896,633, 1983. R.E. Kesting, Synthetic Polymeric Membranes, McGraw-Hill, New York, NY, 1971, p. 29. D. Hillel and S. Gairon, Water transmission rate by a cellulose membrane as related to hydraulic gradient, Isr. J. Chem., 4 (1966) 119.
97
Short Communication
HYDROPHILIZATION
OF POLYETHYLENE
MEMBRANES
A. DIMOV and M.A. ISLAM* Higher Institute (Received
of Chemical Technology,
April 12,1989;
8010 Bow-gas (Bulgaria)
accepted in revised form October 20,1989)
Summary Polyethylene microfiltration membranes were hydrophilized by treating them with ternary mixtures of ethanol-water-inorganic acids. It was found that the threshold pressure necessary to initiate water permeation through the polyethylene membranes depends on the composition of hydrophilizing mixture. The membranes hydrophilized by the ternary solutions showed much better initial flux and lower flux decline rate as compared with those hydrophilized with ethanol alone. It was found that the hydrophilizing efficiency of the inorganic acid in the mixture decreases in the order HNO, > H&30, > H,PO,. However, the flux retention efficiency decreases in the reverse sequence.
Hydrophilization of polyolefin membranes is mostly performed by chemical modification. Well known modification methods are: treatment of the polyolefin membranes with sulphonating agents [ 1 ] and ozone [ 21, or grafting of hydrophilic monomers, such as acrylonitrile [3], acrylic acid [4] and others, onto the polymer macromolecules, so building up the membrane structure. The disadvantage of chemical modification is that, in most cases, it is accompanied by a process of destruction or a change in the porous structure of the membrane. Another kind of hydrophilization is a temporary one, which is achieved by wetting the polyolefin membranes with an organic water-soluble solvent [ 5,6]. After such physical treatment, capillary repulsive forces of the hydrophobic membranes cease to counteract the water flux. The basic disadvantage of this hydrophilization method, in which the chemical structure of the membrane does not change, is that the water flux decreases with time, an effect which is due to gradual leaching of the hydrophilizing agent by the water permeating through the membrane. In the present work, we have attempted to hydrophilize polyethylene membranes by treating them with an alcohol-acid mixture with a view to improve their permeability and working life. Membranes were prepared from a mixture of high-density polyethylene, sil*To whom correspondence
0376-7388/90/$03.50
should be addressed.
0 1990 Elsevier Science Publishers
B.V.
98
icon dioxide and oil. A film with a thickness of 0.4 mm was prepared from the molten polymer composition by pressing. Oil was extracted from the film by treating it with tetrachloroethylene. Membranes with 45% porosity and average pore radius 0.12 pm were formed from the film under the action of thermomechanical deformation. Water flux was determined with a laboratory cell of membrane area 12.56 cm’. Ethanol and a mixture of ethanol and inorganic acids were used as hydrophilizing agents. The membrane samples were immersed in hydrophilizing mixture for 5 min at a temperature of 20-40” C. Polyethylene membranes with average pore radius 0.12 pm do not show any detectable water flux at a pressure of 0.5 MPa. To overcome the capillary repulsive forces towards some non-wetting fluids, a corresponding superatmospheric pressure needs to be applied. Theoretically, according to Cantor’s relationship [ 71, hydrophobic polyethylene membranes with pore radius 0.12 ym are water permeable at a pressure of ca. 1.2 MPa. When the membranes are treated with alcohol, capillary repulsive forces are reduced significantly, and the experimentally determined water flux at a pressure of 0.1 MPa is 0.57 m”_m-2_hr-‘. The water flux as a function of the operating pressure is represented in Fig. 1. The measurements are performed at pressures up to 0.2 MPa in order to avoid membrane compaction, which may cause deviation of the relationship from Darcy’s law. The values of the pressure necessary to initiate water permeation depend on the composition of the hydrophilizing mixture. Some authors [ 81 observe the deviation of the curve from linearity as it approaches the origin. The above mentioned relationship remained linear in our experiments. It is obvious from the figure that, with an increase in the degree of hydrophilization, the curves may be described more accurately by Darcy’s law. At a constant pressure, the highest flux is observed for the membranes treated with a hydrophilizing mixture containing nitric acid.
12
_
0.4 _
Fig. 1. Water flux F as a function of the operating pressure P through membranes treated with different hydrophilizing mixtures: (1) 40 mol% ethanol; (2)) (3) and (4) ternary mixtures of 40 mol% ethanol;
(411.
53.8 mol% water and 6.2 mol% inorganic
acid
[H,PO,
(2), H,SO,
(3), HNOCI
99
06-
Fig. 2. Initial water flux F as a function of the composition of the hydrophilizing mixture; n is the molar fraction of ethanol in the alcohol-water mixture (1) and in the mixtures alcohol-wateinorganic acid [ H,PO, (2 ) , H,SO, (3 ) , HNO, (4) 1.The concentration of acid in all the compositions is 6.2 mol%. Operating pressure 0.1 MPa.
Figure 2 represents the relationship between the initial water flux through the polyethylene membrane and the composition of the hydrophilizing agent, consisting of either a binary ethanol-water mixture or a ternary mixture of water-ethanol-acid. In the ternary mixture, the quantity of the inorganic acid is kept constant (6.2 mol% ) with respect to the other two components (water and ethanol). A low temperature and a low concentration of acid during the treatment ensure the absence of chemical reaction between the membrane material and the hydrophilizing mixture. The results obtained may be explained by the greater affinity of esters than of the corresponding alcohol towards the membrane material. With an increase in the quantity of alcohol, the affinity of the mixture towards the membrane increases. Up to an alcohol concentration of 60 mol% in the ternary mixture, the water flux through the membrane increases quite rapidly, and the incremental rate then slows down. This phenomenon may be explained by the assumption that, beyond the above-mentioned alcohol concentration, a gradual saturation of the hydrophilizing capacity of the mixture takes place and the flux tends to a limiting value. On the other hand, the hydrophobicity follows the order: (CzH5)3P04> ( C,H,)zSO,> C2H5N03, owing to the decreasing contribution of organic ethyl groups in the esters; as a result, permeability increase in the same sequence. The more difficult removal of the hydrophilizing esters than of ethanol is shown in Fig. 3, which represents the water flux as a function of the duration of continuous flow of water through the polyethylene membrane treated with different hydrophilizing mixtures. After an experiment lasting 8 hr under the conditions described at the beginning of this paper, the water flux of membranes treated with mixtures containing nitric acid, sulphuric acid and phos-
Fig. 3. Water flux F as a function
of the duration
r of continuous
flow through the membranes
treated with different hydrophilizingcompositions: 100 mol% ethanol (1); 50 mol% ethanol, 43.8 mol% water, 6.2 mol% H,PO, (2); 40 mol% ethanol, 53.8 mol% water and 6.2 mol% H&SO4 (3) or HN03 (4). Operating pressure 0.15 MPa at entrance and 0.05 MPa at exit.
phoric acid was reduced by 8.7,6.7 and 5.4%, respectively. The temporary hydrophilicity achieved by treatment with the ethanol-water mixture loses 28.1% of its efficiency over the same operating period. It may be concluded that the water flux and the duration of the effective action of the hydrophilized polyethylene membranes may be controlled by changing the composition of the hydrophilizing mixtures.
References K. Matsuda, M. Kohno and Y. Doi, Hydrophilic sulphonated polyolefin porous membrane and process for preparing the same, U.S. Patent 4,409,339,1983. I. Hajima, H. Akira and Y. Toshio, Hydrophilization of porous hydrophilic membranes, Jpn. Patent 6,219,208, 1987. H. Saburo, H. Kumo and K. Yoshisuke, Patent 61,125,409, 1986.
Hydrophilic
polyolefin
hollow fibre membranes,
Jpn.
N. Kan, K. Koichi, M. Kenji, K. Mosato and T. Joichi, Hydrophilic polyethylene microporous membranes, Jpn. Patent 61,106,640, 1986. T.D. Brock, Membrane Filtration, Mir, Moscow, 1987. Mitsubishi Rayon Co. Ltd., Hydrophilic membranes for body fluid filtration, Jpn. Patent 5,896,633, 1983. R.E. Kesting, Synthetic Polymeric Membranes, McGraw-Hill, New York, NY, 1971, p. 29. D. Hillel and S. Gairon, Water transmission rate by a cellulose membrane as related to hydraulic gradient, Isr. J. Chem., 4 (1966) 119.