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Ultrafiltration membranes made of vinyl chloride-vinyl acetate copolymer
Journal of Membrane Science, 76 (1993) 269-280 Elsevier Science Publishers B.V., Amsterdam
269
Ultrafiltration membranes made of vinyl chloride-vinyl acetate copolymer M. Bodzek and K. Konieczny Institute of Engineering and Technology of Water, Sewage and Wastes, Silesian Technical University, Kuczewskiego 2, 44-100 Gliwice (Poland) (Received July 14,1992; accepted in revised form October 8,1992)
Abstract This paper presents the results of investigations concerning the formation, structure and transport properties of tubular ultrafiltration membranes made of copolymer vinyl chloride-vinyl acetate (Winicet, Poland). A method of tubular membrane formation by the phase-inversion method has been worked out. To prepare the casting solution, dimethylformamid was used as the solvent, and water as the nonsolvent. The compactness of the membranes can be controlled by changing the concentration of polymer in the casting solution, the gelation temperature and by using polyethylene glycol as an additive to the casting solution. The porosity of the membranes was determined by means of electron microscopy, the bubble-point method and nitrogen adsorption/desorption method. The obtained membranes are asymmetrical and the macroporous sublayer has a finger-like structure. The pore size distribution of the skin has a rather narrow character. The transport-separation properties of the membranes were determined by measuring the water flux and retention coefficient of water soluble macromolecules and colloids. Keywords: ultrafiltration; membrane preparation and structure; vinyl chloride-vinyl membranes; tubular membranes
Introduction Producers of equipment for ultrafiltration are looking for new polymers for the production of cheap membranes of adequate mechanical strength, thermal and chemical resistance and easy to prepare, but at the same time efficient and selective. Favourable results of our investigations on the production and properties [ 1,2] as well as application [ 3-61 of ultrafiltration membranes Correspondence to: M. Bodzek, Institute of Engineering and Technology of Water, Sewage and Wastes, Silesian Technical University, Kuczewskiego 2, 44-100 Gliwice, Poland.
0376-7388/93/$06.00
acetate copolymer
made of poly (vinyl chloride) (PVC) have prompted us to try to obtain membranes from copolymer vinyl chloride/vinyl acetate (VC/ VAC ) , which are produced by the Polish industry of plastics and are called “Winicet”. The introduction of VAC mers into PVC, functioning as an inner plastifier, raises considerably its elasticity and facilitates the processing of the polymer compared with the homopolymer PVC. VC/VAC copolymers, containing 10% of the latter, are characterized by a doubled impact resistance if compared with that of PVC [ 71. Like PVC, Winicet displays a good chemical resistance to diluted and most concentrated acids [ 81. Moreover, it resists the
0 1993 Elsevier Science Publishers B.V. All rights reserved.
270
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activity of bases, salt solutions, vegetable and mineral oils, alcohols, oxygen and ozone. It is, however, soluble in esters, ketones, aliphatic chlorinated hydrocarbons, aromatic hydrocarbons, pyridine, carbon disulphide and tetrahydrofurane. For the production of foils merely copolymers with Fikentscher’s number 55-60 containing lo-15% VAC are used, with only few exceptions [ 91. But there are no papers dealing with the production of membranes from the copolymer VC/VAC. Our earlier investigations [lo] concerned the determination of the conditions of forming flat ultrafiltration membranes from Winicet as well as their structure and transport and separation properties. The aim of the present paper is to determine adequate conditions for the production of tubular ultrafiltration membranes from the copolymer VC/VAC and to discuss investigations dealing with their structure and functional properties.
Membrane
Sci. 76 (1993) 269-280
lar mass amounted to 82,000 and 35,500, respectively. Preparation of membranes
Materials
For the production of VC/VAC copolymer membranes use was made of the phase-inversion method [7,11], consisting in casting of a film from a polymer solution in some adequate solvent and then gelling it in a non-solvent (water). As a solvent, DMF was used with an addition of polyethylene glycol with a molar mass of 1500 (PEG-1500). Figure 1 shows a diagram of the apparatus in which tubular membranes were obtained. They were formed in an annular gap between walls of glass tube and a self-centering metal bob. When the membrane was formed, the glass tube remained immobile while the metal bob moving upwards formed the membrane along the inner surface of the tube. Such a procedure warrants the production of membranes of a constant thickness (0.2-0.25 mm) along its full length. After the film had been formed, the tube was submerged in a bath of water at the same shift-rate as during its formation.
For our investigations we used Winicet type 50-15G, i.e. a copolymer with 15% of VAC, produced in the Chemical Plant “Oswiecim” by means of suspensive polymerization; according to data provided by the producer [ 81 it is characterized by Fikentscher’s number (K) 49-51. The VC/VAC copolymer applied in the production of membranes was subjected to analysis of the distribution of the molar mass and average molar mass, making use of gel chromatography on the liquid chromatograph ALC/ GRC (Waters Ass. ) . The results of these investigations have been discussed in detail in Ref. [lo]. Attention should be paid to the fact that the polymer sample displayed a rather wide dispersion of the molar mass (D > 2 ) . The values of the weight-average and number-average mo-
Fig. 1. Diagram of the casting system of tubular Winicet membranes. (1) Casting bob, (2) casting tube, (3) fixing ofthe tube, (4) motor, (5) gear, (6) nylonline, (7) control desk.
Experimental
M. Bodzek and R KoniecznyjJ. Membrane Sci. 76 (1993) 269-280
Investigations of membrane structure
The physical structure of the membranes was determined by means of a scanning electron microscope type JSM-35 (Jeol, Japan). The membranes were dehydrated by drying them at reduced pressure (0.133 Pa) and at 313 K, fractured in liquid nitrogen and covered with metallic gold dust in order to obtain a good contrast of the cross-section. The characteristics of the structure of the surface layer and the pore size distribution were determined by means of the bubble-point method [7,12]. Ethyl alcohol was used as the wetting agent and nitrogen as the permeating medium. Measurements were taken in the pressure range of 0.1-0.65 MPa. In the calculations the thickness of the skin layer was assumed to be 5 pm. The microporous structure, i.e. the pore surface area, the volume of the pores and their radii over the whole volume of the membrane, were calculated from adsorption/desorption isotherms of nitrogen at 77 K, making use of Gravimat’s sorption apparatus 4303 (Sartorius). The interpretation of isotherms has been presented in Refs. [ 13,141.
271
Investigations of transport and separation properties
The transport of water and separation of macromolecules and colloids by tubular membranes has been investigated by means of the apparatus of which a diagram as well as its operating principles are shown in Fig. 2. The volumetric water flux was determined by measuring the volume of the water permeating in a given unit of time at a pressure of 0.2 MPa, temperature of 298 K and linear velocity of 3 m/set (while investigating the selection of membrane forming parameters) and 0.05-0.3 MPa. Before measurements, the membranes were conditioned by passing water through them at a pressure of 0.2 MPa for 3 hr. For the measurement of membrane separation properties the container of the apparatus was filled with 10 drn3of test solution and after measurement conditions were reached (pressure 0.2 MPa, temperature 298 K, linear velocity 3 m/set), ultrafiltration was started and carried on for five hours, with return of the permeate. The volume of the recovered permeate was measured, and the content of the substance tested was determined. From these data
Fig. 2. Diagram of the apparatus for the testing of tubular Winicet membranes. (1) Container, (2) heat exchanger, (3) pump, (4) suction conduit, (5) pressure conduit, (6) by-pass conduit, (7) ultrafiltration module, (8) permeate disposal, (9) safety valve, (10) valves, (11) pressure gauge, (12) thermometer, (13) telerotameter.
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the volumetric permeate flux and the retention coefficients were calculated, For testing the following substances were used: polyethylene glycol (PEG): range of molar mass l&000-20,000 (producer: POCh, Gliwice, Poland) dextrane: range of molar mass 4O,OOO110,000 (producer: POCh, Gliwice, Poland) proteins: ovalbumin, albumin and y-globulin (producer: POCh, Gliwice, Poland) as well as an emulsion of ES-oil in water applied in the mechanical treatment of metals as a cooling and lubricating agent, The concentration of PEG in the permeate and test solution was determined gravimetritally, drying the samples at a temperature of 378 K until a solid mass was obtained. The content of dextrane, on the other hand, was determined by means of Bertrand’s titration method [ 151, and the proteins by means of Lowry’s colorimetric method [ 161. The test solutions had the following concentrations: PEG and dextranes: 10 kg/m3, proteins: 0.2 kg/m3. In investigations on the emulsion the concentration of the oil amounted to 10 kg/m3, and the quality of the permeate was evaluated by determining oil concentration calorimetrically in Nessler cylinders, and the chemical oxygen demand (COD) by means of the simplified dichromate method [ 171. Results and discussion Selection of conditions for the formation of membranes As can be seen from Table 1 (A) with the increase of the polymer concentration in the casting solution the volumetric water flux decreases, and membranes made of solutions containing more than 21% of the polymer are impermeable for water, or their permeability is too low from the economic point of view. Mem-
Membrane Sci. 76 (1993) 269-280
TABLE 1 Optimalization of composition of casting solution during formation of tubular membranes made of Winicet (testing substance: water, testing pressure: 0.2 MPa, testing temp.: 298 K, linear velocity over the membrane surface: 3 m/set) (A) Influence of polymer concentration (gelation temp.: 278 K, time of solvent evaporation: 5 set) Concentration (%)
17
18
19
J, (m3/m’-d)
6.52i -3
5.9Oi 3.00’ -3 6.00*
20
21
22
1.501 0.801 -4 4.442 1.36’ 0.50’
(B) Influence of modifier concentration (polymer concentration: 20%, gelation temp.: 278 K, time of solvent evaporation: 5 set) Concentration (%)
0
1.0
2.5
5.0
7.5
10
J, (m3/m2-d)
1.50
2.93
4.47
5.35
5.50
-3
‘Membranes without modifier. ‘Membranes with modifier (25% of PEG-1500 in the casting solution). 3No mechanical resistance. 4No permeability. J, = volume water flux.
branes from casting solutions with a content below 17-18%, on the other hand, are characterized by a heterogeneous surface structure and poor mechanical strength, and in the course of gelling much of polymer was washed out into the gelling bath. Membranes obtained from casting solutions which contain a modifier (polyethylene glycol) are more permeable than membranes obtained from solutions that contain only a polymer and solvent. The addition of a modifier to the casting solution accelerates the gelling of the membrane, favoring the formation of more open structures. The permeability of the membranes grows with the increase of the polyethylene glycol concentration in the casting solution (Table 1B). A modifier content of 2.54% proved to be most favorable. The gelling temperature of the casting solu-
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Membrane Sci. 76 (1993) 269-280
tion influences the rate of the phase-inversion process, and thus also the structure of the membranes obtained [X3-20]. Membranes obtained at lower temperature are more permeable, mechanically more resistant and have a more homogeneous surface structure, because the gelling process is slower (Table 2A). No influence of the time of evaporation of the solvent upon the compactness of the obtained membranes has been detected, probably due to the weak volatility (high boiling point ) of the solvent, i.e. DMF (Table 2B ) . In summary it may be said that tubular Winicet membranes should be obtained applying the following process conditions and parameters: concentration of the polymer: 17-2‘2% concentration of the modifier (polyethylene glycol with a molar mass of 1500) (if necessary): 2.55% temperature of the casting solution and the temperature of casting: 293-298 K time of evaporation of the solvent: 5-30 set gelling agent: water TABLE 2 Optimalization of some formation conditions of tubular membranes made of Winicet (testing substance: water, testing pressure: 0.2 MPa, testing temp.: 298 K, linear velocity over the membrane surface: 3 m/set) (A) Influence of gelation temperature (time of solvent evaporation: 5 set) Gelation temperature (K )
278
J, (m3/m2-d)
6.401 4.401 4.002 3.152
295
(B) Influence of solvent evaporation time2 (gelation temp.: 278 K) Solvent evaporation time (set)
5
30
60
120
J, (m3/m2-d)
440
447
424
408
‘Membranes without modifier (20% of polymer). 2Membranes with modifier (18% of polymer and 2.5% of PEG-1500 in the casting solution). J, = volume water flux.
273
- temperature of the gelling bath: 278 K - minimum time of gelling: 900 set - width of the gap in the course of gelling: 0.20.25 mm. Membranes obtained under such conditions are characterized by a volumetric water flux of 0.5-6.5 m3/m2-d. The compactness of the membranes is mainly controlled by adjusting the concentration of the polymer in the casting solution, by adding a modified and by checking the temperature during membrane casting. Porous structure of membranes Figure 3 contains micrographs of the crosssections of membranes obtained from vinyl chloride-vinyl acetate copolymer. An analysis of these photos leads to the conclusion that the tubular membranes are asymmetrical in their structure, consisting of a very thin skin layer and a macroporous sublayer. Membranes obtained from solutions with a low concentration have a sublayer which contains voids (macropores) constituting a considerable part of it (Figs. 3a and 3b). With increasing concentration the volume of the voids becomes smaller (Fig. 3~). No distinct difference has been discovered in the structure of the porous sublayer of the membrane obtained from casting solutions with different concentrations. The differences in the compactness of the three membranes result mainly from differences in their thickness. In spite of the fact that while casting the film, the height of the gap was kept constant (about 0.2 mm), the obtained membranes differ in their thickness due to the differing mass of the polymer contained in the same volume of the casting solution. The thickness of the membranes as measured from micrographs of their cross-section amounts to 6385 pm, depending on the concentration of the polymer in the casting solution. In Fig. 4 we can see cross-sections of membranes obtained from casting solutions which,
M. Bodzek and K. KoniecznylJ Membrane Sci. 76 (1993) 269-280
Fig. 3. Microphotographs of the cross-sections of tubular membranes obtained from solutions containing Winicet in DMFat aconcentrationof 18% (a), 19% (b) and20% (c) (1 cm = 56nm).
Fig. 4. Microphotographs of the cross-sections of tubular membranes obtained from solutions containing DMF, PEG-1500 and Winicet at concentrations of 19% (a), 20% (b) and21% (c) (1 cm = 56pm).
M. Bodzek and K. KoniecznyjJ. Membrane Sci. 76 (1993) 269-280
in addition to the polymer and DMF, contained also a modifier. They display an asymmetrical structure and a structure of the macroporous sublayer somewhat similar to the case of membranes obtained without a modifier. The thickness of the membranes and volume of the voids are strictly connected with the concentration of polymer in the casting solution. If we compare the structure of membranes obtained from casting solutions containing an identical amount of polymer with and without a modifier (membranes W-19, W-19PEG and W-20, W-BOPEG), we find that the structure of the latter ones is less dense and the volume of the voids is larger. Analysis of the micrographs showing the cross-sections of tubular VC/VAC copolymer membranes (Figs. 3,4) leads to the conclusion that the structure of voids (macropores) in the membrane sublayer differs in the respective parts of the cross-section. In the upper part (2030% of the cross-section) there are finger-like structured macropores with small diameters. The bottom part, however, contains voids of a
Fig. 5. Distribution
275
similar shape but with larger diameters. Between the voids the structure is spongy. The formation of the asymmetrically structured membranes might be explained to be the result of a varying rate of the phase-inversion process in the course of gelling [ 18-201. If the removal of DMF from the cast film runs quicker, a more open structure with larger pores is formed (Figs. 3A and 4A). Moreover, of much importance is the structure of the skin layer, because it restricts the diffusion of the non-solvent in both directions of the cast film. The velocity of diffusion differs depending on the permeability and thickness of this layer. The thickness of the skin layer increases with an increase in polymer concentration in the casting solution [ 211. Membranes obtained from solutions with a lower concentration have a thinner skin layer, resulting in a more open structure. In the case of membranes obtained from solutions with higher polymer concentrations more compact structures are being formed. Figures 5 and 6 show the pore characteristics
of pore radii in the skin layer of tubular Winicet membranes
symbol denotes the percentage
of polymer content in the casting solution)
(the number at the end of the membrane
by the bubble-point
method.
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M. Bodzek and K. KoniecznyjJ.
r - PORCRADIUB
W-20
Membrane Sci. 76 (1993) 269-280
(nm)
PEG
Fig. 6. Distribution of pore radii in the skin layer of tubular Winicet membranes obtained from casting solutions containing an addition of 2.5% PEG-1500 (the number at the end of the membrane symbol denotes the percentage of polymer content in the casting solution) by the bubble-point method.
of the skin layer for selected membranes obtained by the bubble-point method. The average pore radius in the skin layer decreases with an increase of the polymer concentration in the casting solution, which means that the obtained membranes become more compact. This has been confirmed by the results of water permeability through the membranes. Figures 5 and 6 illustrate also the distribution of pore sizes in the skin layer. Both membranes obtained from casting solutions with and without modifier have sharp maxima of pores with similar radii. For the respective membranes the following values have been found: - W-17: 79% of pores fall in the range 213-224 nm, - W-18: 69% of pores fall in the range 64.770.3 nm, - W-19: 79% of pores fall in the range 50.5 55.5 nm, - W-19PEG: 87% of pores fall in the range 6066.5 nm, - W-20PEG: 95.5% of pores fall in the range 44-48 nm. For the investigation of the porous structure
of the membranes by the adsorption/desorption of nitrogen, four tubular membranes without any modifier were used (W-17, W-18, W19 and W-20, respectively), as well as two membranes obtained from solutions containing 2.5% polyethylene glycol (W-19PEG and W-20PEG, respectively). The adsorption and desorption curves coincide for all membranes. The fact that no hysteresis occurs between the adsorption and desorption isotherms proves that the membranes have a structure in which the pores are not open capillaries. Observations made by electron microscopy (Figs. 3 and 4) have shown that Winicet membranes contain large “finger-like” macropores. In such membranes the components of the solutions are being transported inside a porous spongy structure. Table 3 shows the pore surface area and other parameters of the pore structure of the membranes [ 13,14,22]. The surface area of the pores increases with the polymer concentration in the casting solution due to the increase of the number of smaller pores. The surface area is similar for membranes obtained from casting
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Membrane Sci. 76 (1993) 269-280
TABLE 3 Porous structure characterization of tubular ultrafiltration membranes made of Winicet Membrane symbol
Surface area (m*/g)
Pore volume (cm3/g)
Mean pore radius (nm )
w-17 W-18 w-19 w-20 W-19PEG W-ZOPEG
7.94 8.57 9.91 10.9 10.8 10.9
0.0090 0.0110 0.0116 0.0140 0.0127 0.0132
2.27 2.57 2.34 2.57 2.34 2.67
solutions with an identical polymer concentration, independent of the presence or absence of a modifier. The volume of the pores also grows with an increase in polymer concentration of the casting solution. Somewhat larger pore volumes were obtained in the case of membranes obtained from casting solutions containing a modifier. The experimentally found volume of the pores amounts to 0.009-0.014 cm3/g, depending on the polymer concentration in the casting solution. The values of the mean radius of the pores are rather similar and rather small (2.27-2.67 nm). Their value is a rather hypothetical one. The differences between pore size determinations made by the bubble-point method and the gas adsorption/desorption method are related with the differences between the skin layer and sublayer structure. With the bubble-point method the skin layer pore structure is determined, while the nitrogen adsorption/desorption method encompasses the whole structure (skin layer and sublayer ) [ 23 1.
Transport and separation properties Transport and separation properties have been determined for three selected tubular Winicet membranes, differing from each other with respect to their compactness:
277
membrane I (compact - prepared from casting solution containing 19% of polymer and without modifier): water flux at 0.2 MPa = ca. 2.5 m3/m2-d, membrane II (intermediate compactness prepared from casting solution containing 18% of polymer and without modifier): water flux at 0.2 MPa = ca. 5 m3/m2-d, membrane III (open - prepared from casting solution containing 17% of polymer and without modifier): water flux at 0.2 MPa = ca. 7 m3/m2-d. The dependence of the volumetric water flux (J,) on pressure (AP) for three membranes can be described rather accurately both by the linear function: J, =A,AP and the potential function: J,=A:,
(AP)*
where A, is the permeability of the membrane with respect to water, AL is the regression constant of the potential equation, and b is the exponent of the potential function. Table 4 shows the values of the equation constants obtained from experimental data. The coefficients of correlation and determination resulting from the approximation of the experimental data and the respective equations were very similar to each other. TABLE 4 Values of A,,,, A & and b in equations describing the dependence of volume water flux (J,) on pressure (dP) [J, =A, dPand J,=A& AP*] for tubular membranes made of Winicet (J,., is expressed in m/d and AP in MPa) Membrane type
A,
Dense membrane I Medium porosity membrane II Open membrane III
12.5
A:, 7.00
b 0.622
21.8
10.4
0.507
31.6
20.1
0.691
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TABLE 5 Transport-separation properties of tubular membranes made of Winicet (testing pressure: 0.2 MPa, testing temp.: 298 K, linear velocity over the membrane surface: 3 m/set) Testing substances
Molecular weight
Membrane I Permeate
Membrane II R
flUX
Dextran Dextran Dextran Polyethylene glycol Ovalbumin Albumin y-Globulin EN oil emulsion
40,000 70,000 110,000 15,000 20,000 42,000 69,000 160,000
Permeate
Membrane III R
flux
Permeate
R
flux
(m3/m”-d)
(%)
(m3/m2-d)
(% )
(m3/m2-d)
(%)
1.36 1.52 0.990 1.34 1.27 1.52 1.28 1.24 1.52
82.0 88.0 95.0 22.4 59.1 90.0 92.5 100 92.0’197.02
2.24 2.12 2.08 2.08 2.04 2.12 2.00 1.44 2.08
70.0 81.1 95.0 21.0 51.8 90.0 86.7 100 90.0’/96.8’
2.92 2.43 2.40 3.04 2.88 3.24 2.54 2.48 2.40
52.0 87.0 95.0 20.0 23.0 53.2 87.5 100 80.0’/95.82
‘Retention coefficient of turbidity. 2Retention coefficient of COD. R = retention coefficient.
Table 5 contains the volumetric permeate flux and the experimental retention coefficients obtained for various Winicet membranes differing in their compactness and for various feeds. The volumetric permeate flux decreases with an increase of the molar mass of the permeant and with increasing compactness of the membrane with given macromolecule and emulsion. The most retained substances are particles of oil emulsions in water and proteins. The retention coefficients varied from 100% to 80% depending on membrane compactness and molar mass of the proteins. Dextranes were retained less effectively, though still to a high degree. Dextran with a molar mass of 110,000, for instance, was retained by all the membranes for 95%, whereas the retention coefficient of dextranes with lower molar mass varied within the range 81438% and 52-82%, respectively, depending on the compactness of the membrane. The lowest retention was observed in the case of polyethylene glycols, due to the linear structure of their molecules.
In summary, it may be said that the retention coefficient increases with increasing compactness of the membrane, although the degree of this dependence varies from one test substance to another. Similar dependencies as those mentioned above have been observed earlier in the course of our investigations on polyacrylonitrile-poly (vinyl chloride) membranes [ 1,2] as well as on flat Winicet membranes [lo]. Conclusions (1) Winicet - vinyl chloride-vinyl acetate copolymer - may be used in the production of tubular ultrafiltration membranes by means of the phase-inversion method, applying dimethylformamide as a solvent and water as a precipitating bath. (2) By changing the concentration of the polymer in the casting solution and adding polyethylene glycol to the casting solution membranes of different compactness can be obThe compactness of Winicet tained.
M. Bodzek and K. KoniecznyjJ. Membrane Sci. 76 (1993) 269-280
membranes is also affected by the temperature of gelling of the cast film. (3) The obtained Winicet membranes display an asymmetrical structure, the structure of the membrane sublayer being of the “fingerlike” type with increasing macropore sizes depending on their distance from the skin layer. (4) The skin layer of tubular Winicet membranes is characterized by a narrow range of pore sizes. In the case of membranes obtained from Winicet solutions with a concentration of M-20%, these values range from 50 to 70 nm. (5 ) The nitrogen adsorption/desorption isotherms obtained for tubular Winicet membranes do not display any hysteresis, which proves that no open capillaries are being formed. (6) The surface area of the pores as well as their volume depend on the compactness of the membranes. (7) The dependence of the volumetric water flux (J,) on pressure (AP) is described both by the linear function J,=A,dP and the potential function J, =A L Mb, the permeability of the membrane (A, and A& ) decreasing with increasing compactness and assuming a value of 0.5-0.7 for the potential exponent. (8) Tubular Winicet membranes retain yglobulin as well as colloidal substances completely, and to a high extent also other proteins and dextranes with a molecular mass above 20,000.
References M. Bodzek, Ultrafiltration membranes made of polyacrylonitrile and poly (vinyl chloride ) , in : B. Sedlacek and J. Kahovec (Eds.), Synthetic Polymeric Membranes, W. de Gruyter, Berlin, 1987, pp. 193-202. M. Bodzek and K. Konieczny, The influence of molecular mass of poly (vinyl chloride) on the structure and transport characteristics of ultrafiltration membranes, J. Membrane Sci., 61 (1991) 131. M. Bodzek, K. Konieczny and I. Tanistra, Poly (vinyl chloride) membranes in the treatment of waters and wastewaters, Chemia Stosowana, 32 (1988) 311. M. Bodzek, J. Bohdziewicz, I. Tanistra and K. Kon-
279
ieczny, Application of poly(viny1 chloride) and polyacrylonitrile membranes to concentration, purification and fractionation of macromolecules and colloids, Environ. Protect. Eng., 15 (1989) 121. 5 K. Konieczny and M. Bodzek, Ultrafiltration of water containing colloidal silica, Desalination, 79 (1990) 145. 6 M. Bodzek and K. Konieczny, Polyacrylonitrile and poly(viny1 chloride) ultrafiltration membranes in the treatment of anaphoretic emulsion paint, Desalination, 80 (1991) 43. 7 R.E. Kesting, Synthetic Polymeric Membranes - A Structural Perspective, John Wiley & Sons, New York, NY, 1985. 8 “Chemiplast Erg”, Gliwice, Poland (prospect). 9 Anon., Kunststoff Handbuch Band II, Polyvinylchlorid, Carl Hauser Verlag, Miinchen, 1963. 10 M. Bodzek and K. Konieczny, Ultrafiltration membranes made of Winicet, Polimery, 36 (1991) 233 (in Polish ) . 11 S. Loeb and S. Sourirajan, High-flow semipermeable membranes for separation of water from saline solutions, Adv. Chem. Ser., 38 (1961) 117. 12 W. Kujawski, P. Adamczyk and A. Narcbska, A fully automated system for determination of pore size distribution in microfiltration and ultrafiltration membranes, Sep. Sci. Technol., 24 (1989) 495. 13 J. Broeknoff. Adsorption and capillarity, Ph.D. Thesis, Delft, 1969. 14 M. Bodzek, Determination of pore characteristics of polymer ultrafiltration membranes by gas adsorption/desorption method, Chemia Analityczna, 32 (1985) 563. 15
16
17 18
19
20
21
22 23
R.L. Whistler and J.N. Miller (Eds.), Methods in Carbohydrate Chemistry, Academic Press, New York, NY, 1962. H. Lowry, N.J. Rosenbrough, A.L. Farr and R.J. Randal, Protein measurements with the folin-phenol reagent, J. Biol. Chem., 193 (1951) 261. J.S. Jeris, A rapid COD test, Water Wastes Eng., 4 (1967) 89. H. Strathmann and K. Kock, The formation mechanism of phase-inversion membranes, Desalination, 21 (1977) 241. H. Strathmann and 0. Scheible, Zum Bildungsmechanismus von asymmetrischen Zellulose-acetat Membranen, Kolloid Z.u.Z. Polym., 246 (1971) 669. M. Bodzek, Formation mechanism of asymmetric ultrafiltration membranes, Wiad. Chem., 40 (1986) 101 (in Polish). M. Bodzek, Physico-chemical characterization of ultrafiltration membranes, Polish J. Chem., 57 (1983) 919. S.J. Gregg and K.S. King, Adsorption, Surface Area and Porosity, Izd. Mir, Moskow, 1970. M. Bodzek and J. Bohdziewicz, Porous polycarbonate phase-inversion membranes, J. Membrane Sci., 60 (1991) 25.
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Ultrafiltration membranes made of vinyl chloride-vinyl acetate copolymer M. Bodzek and K. Konieczny Institute of Engineering and Technology of Water, Sewage and Wastes, Silesian Technical University, Kuczewskiego 2, 44-100 Gliwice (Poland) (Received July 14,1992; accepted in revised form October 8,1992)
Abstract This paper presents the results of investigations concerning the formation, structure and transport properties of tubular ultrafiltration membranes made of copolymer vinyl chloride-vinyl acetate (Winicet, Poland). A method of tubular membrane formation by the phase-inversion method has been worked out. To prepare the casting solution, dimethylformamid was used as the solvent, and water as the nonsolvent. The compactness of the membranes can be controlled by changing the concentration of polymer in the casting solution, the gelation temperature and by using polyethylene glycol as an additive to the casting solution. The porosity of the membranes was determined by means of electron microscopy, the bubble-point method and nitrogen adsorption/desorption method. The obtained membranes are asymmetrical and the macroporous sublayer has a finger-like structure. The pore size distribution of the skin has a rather narrow character. The transport-separation properties of the membranes were determined by measuring the water flux and retention coefficient of water soluble macromolecules and colloids. Keywords: ultrafiltration; membrane preparation and structure; vinyl chloride-vinyl membranes; tubular membranes
Introduction Producers of equipment for ultrafiltration are looking for new polymers for the production of cheap membranes of adequate mechanical strength, thermal and chemical resistance and easy to prepare, but at the same time efficient and selective. Favourable results of our investigations on the production and properties [ 1,2] as well as application [ 3-61 of ultrafiltration membranes Correspondence to: M. Bodzek, Institute of Engineering and Technology of Water, Sewage and Wastes, Silesian Technical University, Kuczewskiego 2, 44-100 Gliwice, Poland.
0376-7388/93/$06.00
acetate copolymer
made of poly (vinyl chloride) (PVC) have prompted us to try to obtain membranes from copolymer vinyl chloride/vinyl acetate (VC/ VAC ) , which are produced by the Polish industry of plastics and are called “Winicet”. The introduction of VAC mers into PVC, functioning as an inner plastifier, raises considerably its elasticity and facilitates the processing of the polymer compared with the homopolymer PVC. VC/VAC copolymers, containing 10% of the latter, are characterized by a doubled impact resistance if compared with that of PVC [ 71. Like PVC, Winicet displays a good chemical resistance to diluted and most concentrated acids [ 81. Moreover, it resists the
0 1993 Elsevier Science Publishers B.V. All rights reserved.
270
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activity of bases, salt solutions, vegetable and mineral oils, alcohols, oxygen and ozone. It is, however, soluble in esters, ketones, aliphatic chlorinated hydrocarbons, aromatic hydrocarbons, pyridine, carbon disulphide and tetrahydrofurane. For the production of foils merely copolymers with Fikentscher’s number 55-60 containing lo-15% VAC are used, with only few exceptions [ 91. But there are no papers dealing with the production of membranes from the copolymer VC/VAC. Our earlier investigations [lo] concerned the determination of the conditions of forming flat ultrafiltration membranes from Winicet as well as their structure and transport and separation properties. The aim of the present paper is to determine adequate conditions for the production of tubular ultrafiltration membranes from the copolymer VC/VAC and to discuss investigations dealing with their structure and functional properties.
Membrane
Sci. 76 (1993) 269-280
lar mass amounted to 82,000 and 35,500, respectively. Preparation of membranes
Materials
For the production of VC/VAC copolymer membranes use was made of the phase-inversion method [7,11], consisting in casting of a film from a polymer solution in some adequate solvent and then gelling it in a non-solvent (water). As a solvent, DMF was used with an addition of polyethylene glycol with a molar mass of 1500 (PEG-1500). Figure 1 shows a diagram of the apparatus in which tubular membranes were obtained. They were formed in an annular gap between walls of glass tube and a self-centering metal bob. When the membrane was formed, the glass tube remained immobile while the metal bob moving upwards formed the membrane along the inner surface of the tube. Such a procedure warrants the production of membranes of a constant thickness (0.2-0.25 mm) along its full length. After the film had been formed, the tube was submerged in a bath of water at the same shift-rate as during its formation.
For our investigations we used Winicet type 50-15G, i.e. a copolymer with 15% of VAC, produced in the Chemical Plant “Oswiecim” by means of suspensive polymerization; according to data provided by the producer [ 81 it is characterized by Fikentscher’s number (K) 49-51. The VC/VAC copolymer applied in the production of membranes was subjected to analysis of the distribution of the molar mass and average molar mass, making use of gel chromatography on the liquid chromatograph ALC/ GRC (Waters Ass. ) . The results of these investigations have been discussed in detail in Ref. [lo]. Attention should be paid to the fact that the polymer sample displayed a rather wide dispersion of the molar mass (D > 2 ) . The values of the weight-average and number-average mo-
Fig. 1. Diagram of the casting system of tubular Winicet membranes. (1) Casting bob, (2) casting tube, (3) fixing ofthe tube, (4) motor, (5) gear, (6) nylonline, (7) control desk.
Experimental
M. Bodzek and R KoniecznyjJ. Membrane Sci. 76 (1993) 269-280
Investigations of membrane structure
The physical structure of the membranes was determined by means of a scanning electron microscope type JSM-35 (Jeol, Japan). The membranes were dehydrated by drying them at reduced pressure (0.133 Pa) and at 313 K, fractured in liquid nitrogen and covered with metallic gold dust in order to obtain a good contrast of the cross-section. The characteristics of the structure of the surface layer and the pore size distribution were determined by means of the bubble-point method [7,12]. Ethyl alcohol was used as the wetting agent and nitrogen as the permeating medium. Measurements were taken in the pressure range of 0.1-0.65 MPa. In the calculations the thickness of the skin layer was assumed to be 5 pm. The microporous structure, i.e. the pore surface area, the volume of the pores and their radii over the whole volume of the membrane, were calculated from adsorption/desorption isotherms of nitrogen at 77 K, making use of Gravimat’s sorption apparatus 4303 (Sartorius). The interpretation of isotherms has been presented in Refs. [ 13,141.
271
Investigations of transport and separation properties
The transport of water and separation of macromolecules and colloids by tubular membranes has been investigated by means of the apparatus of which a diagram as well as its operating principles are shown in Fig. 2. The volumetric water flux was determined by measuring the volume of the water permeating in a given unit of time at a pressure of 0.2 MPa, temperature of 298 K and linear velocity of 3 m/set (while investigating the selection of membrane forming parameters) and 0.05-0.3 MPa. Before measurements, the membranes were conditioned by passing water through them at a pressure of 0.2 MPa for 3 hr. For the measurement of membrane separation properties the container of the apparatus was filled with 10 drn3of test solution and after measurement conditions were reached (pressure 0.2 MPa, temperature 298 K, linear velocity 3 m/set), ultrafiltration was started and carried on for five hours, with return of the permeate. The volume of the recovered permeate was measured, and the content of the substance tested was determined. From these data
Fig. 2. Diagram of the apparatus for the testing of tubular Winicet membranes. (1) Container, (2) heat exchanger, (3) pump, (4) suction conduit, (5) pressure conduit, (6) by-pass conduit, (7) ultrafiltration module, (8) permeate disposal, (9) safety valve, (10) valves, (11) pressure gauge, (12) thermometer, (13) telerotameter.
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the volumetric permeate flux and the retention coefficients were calculated, For testing the following substances were used: polyethylene glycol (PEG): range of molar mass l&000-20,000 (producer: POCh, Gliwice, Poland) dextrane: range of molar mass 4O,OOO110,000 (producer: POCh, Gliwice, Poland) proteins: ovalbumin, albumin and y-globulin (producer: POCh, Gliwice, Poland) as well as an emulsion of ES-oil in water applied in the mechanical treatment of metals as a cooling and lubricating agent, The concentration of PEG in the permeate and test solution was determined gravimetritally, drying the samples at a temperature of 378 K until a solid mass was obtained. The content of dextrane, on the other hand, was determined by means of Bertrand’s titration method [ 151, and the proteins by means of Lowry’s colorimetric method [ 161. The test solutions had the following concentrations: PEG and dextranes: 10 kg/m3, proteins: 0.2 kg/m3. In investigations on the emulsion the concentration of the oil amounted to 10 kg/m3, and the quality of the permeate was evaluated by determining oil concentration calorimetrically in Nessler cylinders, and the chemical oxygen demand (COD) by means of the simplified dichromate method [ 171. Results and discussion Selection of conditions for the formation of membranes As can be seen from Table 1 (A) with the increase of the polymer concentration in the casting solution the volumetric water flux decreases, and membranes made of solutions containing more than 21% of the polymer are impermeable for water, or their permeability is too low from the economic point of view. Mem-
Membrane Sci. 76 (1993) 269-280
TABLE 1 Optimalization of composition of casting solution during formation of tubular membranes made of Winicet (testing substance: water, testing pressure: 0.2 MPa, testing temp.: 298 K, linear velocity over the membrane surface: 3 m/set) (A) Influence of polymer concentration (gelation temp.: 278 K, time of solvent evaporation: 5 set) Concentration (%)
17
18
19
J, (m3/m’-d)
6.52i -3
5.9Oi 3.00’ -3 6.00*
20
21
22
1.501 0.801 -4 4.442 1.36’ 0.50’
(B) Influence of modifier concentration (polymer concentration: 20%, gelation temp.: 278 K, time of solvent evaporation: 5 set) Concentration (%)
0
1.0
2.5
5.0
7.5
10
J, (m3/m2-d)
1.50
2.93
4.47
5.35
5.50
-3
‘Membranes without modifier. ‘Membranes with modifier (25% of PEG-1500 in the casting solution). 3No mechanical resistance. 4No permeability. J, = volume water flux.
branes from casting solutions with a content below 17-18%, on the other hand, are characterized by a heterogeneous surface structure and poor mechanical strength, and in the course of gelling much of polymer was washed out into the gelling bath. Membranes obtained from casting solutions which contain a modifier (polyethylene glycol) are more permeable than membranes obtained from solutions that contain only a polymer and solvent. The addition of a modifier to the casting solution accelerates the gelling of the membrane, favoring the formation of more open structures. The permeability of the membranes grows with the increase of the polyethylene glycol concentration in the casting solution (Table 1B). A modifier content of 2.54% proved to be most favorable. The gelling temperature of the casting solu-
M. Bodzek and K. Konieczny/J.
Membrane Sci. 76 (1993) 269-280
tion influences the rate of the phase-inversion process, and thus also the structure of the membranes obtained [X3-20]. Membranes obtained at lower temperature are more permeable, mechanically more resistant and have a more homogeneous surface structure, because the gelling process is slower (Table 2A). No influence of the time of evaporation of the solvent upon the compactness of the obtained membranes has been detected, probably due to the weak volatility (high boiling point ) of the solvent, i.e. DMF (Table 2B ) . In summary it may be said that tubular Winicet membranes should be obtained applying the following process conditions and parameters: concentration of the polymer: 17-2‘2% concentration of the modifier (polyethylene glycol with a molar mass of 1500) (if necessary): 2.55% temperature of the casting solution and the temperature of casting: 293-298 K time of evaporation of the solvent: 5-30 set gelling agent: water TABLE 2 Optimalization of some formation conditions of tubular membranes made of Winicet (testing substance: water, testing pressure: 0.2 MPa, testing temp.: 298 K, linear velocity over the membrane surface: 3 m/set) (A) Influence of gelation temperature (time of solvent evaporation: 5 set) Gelation temperature (K )
278
J, (m3/m2-d)
6.401 4.401 4.002 3.152
295
(B) Influence of solvent evaporation time2 (gelation temp.: 278 K) Solvent evaporation time (set)
5
30
60
120
J, (m3/m2-d)
440
447
424
408
‘Membranes without modifier (20% of polymer). 2Membranes with modifier (18% of polymer and 2.5% of PEG-1500 in the casting solution). J, = volume water flux.
273
- temperature of the gelling bath: 278 K - minimum time of gelling: 900 set - width of the gap in the course of gelling: 0.20.25 mm. Membranes obtained under such conditions are characterized by a volumetric water flux of 0.5-6.5 m3/m2-d. The compactness of the membranes is mainly controlled by adjusting the concentration of the polymer in the casting solution, by adding a modified and by checking the temperature during membrane casting. Porous structure of membranes Figure 3 contains micrographs of the crosssections of membranes obtained from vinyl chloride-vinyl acetate copolymer. An analysis of these photos leads to the conclusion that the tubular membranes are asymmetrical in their structure, consisting of a very thin skin layer and a macroporous sublayer. Membranes obtained from solutions with a low concentration have a sublayer which contains voids (macropores) constituting a considerable part of it (Figs. 3a and 3b). With increasing concentration the volume of the voids becomes smaller (Fig. 3~). No distinct difference has been discovered in the structure of the porous sublayer of the membrane obtained from casting solutions with different concentrations. The differences in the compactness of the three membranes result mainly from differences in their thickness. In spite of the fact that while casting the film, the height of the gap was kept constant (about 0.2 mm), the obtained membranes differ in their thickness due to the differing mass of the polymer contained in the same volume of the casting solution. The thickness of the membranes as measured from micrographs of their cross-section amounts to 6385 pm, depending on the concentration of the polymer in the casting solution. In Fig. 4 we can see cross-sections of membranes obtained from casting solutions which,
M. Bodzek and K. KoniecznylJ Membrane Sci. 76 (1993) 269-280
Fig. 3. Microphotographs of the cross-sections of tubular membranes obtained from solutions containing Winicet in DMFat aconcentrationof 18% (a), 19% (b) and20% (c) (1 cm = 56nm).
Fig. 4. Microphotographs of the cross-sections of tubular membranes obtained from solutions containing DMF, PEG-1500 and Winicet at concentrations of 19% (a), 20% (b) and21% (c) (1 cm = 56pm).
M. Bodzek and K. KoniecznyjJ. Membrane Sci. 76 (1993) 269-280
in addition to the polymer and DMF, contained also a modifier. They display an asymmetrical structure and a structure of the macroporous sublayer somewhat similar to the case of membranes obtained without a modifier. The thickness of the membranes and volume of the voids are strictly connected with the concentration of polymer in the casting solution. If we compare the structure of membranes obtained from casting solutions containing an identical amount of polymer with and without a modifier (membranes W-19, W-19PEG and W-20, W-BOPEG), we find that the structure of the latter ones is less dense and the volume of the voids is larger. Analysis of the micrographs showing the cross-sections of tubular VC/VAC copolymer membranes (Figs. 3,4) leads to the conclusion that the structure of voids (macropores) in the membrane sublayer differs in the respective parts of the cross-section. In the upper part (2030% of the cross-section) there are finger-like structured macropores with small diameters. The bottom part, however, contains voids of a
Fig. 5. Distribution
275
similar shape but with larger diameters. Between the voids the structure is spongy. The formation of the asymmetrically structured membranes might be explained to be the result of a varying rate of the phase-inversion process in the course of gelling [ 18-201. If the removal of DMF from the cast film runs quicker, a more open structure with larger pores is formed (Figs. 3A and 4A). Moreover, of much importance is the structure of the skin layer, because it restricts the diffusion of the non-solvent in both directions of the cast film. The velocity of diffusion differs depending on the permeability and thickness of this layer. The thickness of the skin layer increases with an increase in polymer concentration in the casting solution [ 211. Membranes obtained from solutions with a lower concentration have a thinner skin layer, resulting in a more open structure. In the case of membranes obtained from solutions with higher polymer concentrations more compact structures are being formed. Figures 5 and 6 show the pore characteristics
of pore radii in the skin layer of tubular Winicet membranes
symbol denotes the percentage
of polymer content in the casting solution)
(the number at the end of the membrane
by the bubble-point
method.
276
M. Bodzek and K. KoniecznyjJ.
r - PORCRADIUB
W-20
Membrane Sci. 76 (1993) 269-280
(nm)
PEG
Fig. 6. Distribution of pore radii in the skin layer of tubular Winicet membranes obtained from casting solutions containing an addition of 2.5% PEG-1500 (the number at the end of the membrane symbol denotes the percentage of polymer content in the casting solution) by the bubble-point method.
of the skin layer for selected membranes obtained by the bubble-point method. The average pore radius in the skin layer decreases with an increase of the polymer concentration in the casting solution, which means that the obtained membranes become more compact. This has been confirmed by the results of water permeability through the membranes. Figures 5 and 6 illustrate also the distribution of pore sizes in the skin layer. Both membranes obtained from casting solutions with and without modifier have sharp maxima of pores with similar radii. For the respective membranes the following values have been found: - W-17: 79% of pores fall in the range 213-224 nm, - W-18: 69% of pores fall in the range 64.770.3 nm, - W-19: 79% of pores fall in the range 50.5 55.5 nm, - W-19PEG: 87% of pores fall in the range 6066.5 nm, - W-20PEG: 95.5% of pores fall in the range 44-48 nm. For the investigation of the porous structure
of the membranes by the adsorption/desorption of nitrogen, four tubular membranes without any modifier were used (W-17, W-18, W19 and W-20, respectively), as well as two membranes obtained from solutions containing 2.5% polyethylene glycol (W-19PEG and W-20PEG, respectively). The adsorption and desorption curves coincide for all membranes. The fact that no hysteresis occurs between the adsorption and desorption isotherms proves that the membranes have a structure in which the pores are not open capillaries. Observations made by electron microscopy (Figs. 3 and 4) have shown that Winicet membranes contain large “finger-like” macropores. In such membranes the components of the solutions are being transported inside a porous spongy structure. Table 3 shows the pore surface area and other parameters of the pore structure of the membranes [ 13,14,22]. The surface area of the pores increases with the polymer concentration in the casting solution due to the increase of the number of smaller pores. The surface area is similar for membranes obtained from casting
M. Bodzek and K. Konieczny/J.
Membrane Sci. 76 (1993) 269-280
TABLE 3 Porous structure characterization of tubular ultrafiltration membranes made of Winicet Membrane symbol
Surface area (m*/g)
Pore volume (cm3/g)
Mean pore radius (nm )
w-17 W-18 w-19 w-20 W-19PEG W-ZOPEG
7.94 8.57 9.91 10.9 10.8 10.9
0.0090 0.0110 0.0116 0.0140 0.0127 0.0132
2.27 2.57 2.34 2.57 2.34 2.67
solutions with an identical polymer concentration, independent of the presence or absence of a modifier. The volume of the pores also grows with an increase in polymer concentration of the casting solution. Somewhat larger pore volumes were obtained in the case of membranes obtained from casting solutions containing a modifier. The experimentally found volume of the pores amounts to 0.009-0.014 cm3/g, depending on the polymer concentration in the casting solution. The values of the mean radius of the pores are rather similar and rather small (2.27-2.67 nm). Their value is a rather hypothetical one. The differences between pore size determinations made by the bubble-point method and the gas adsorption/desorption method are related with the differences between the skin layer and sublayer structure. With the bubble-point method the skin layer pore structure is determined, while the nitrogen adsorption/desorption method encompasses the whole structure (skin layer and sublayer ) [ 23 1.
Transport and separation properties Transport and separation properties have been determined for three selected tubular Winicet membranes, differing from each other with respect to their compactness:
277
membrane I (compact - prepared from casting solution containing 19% of polymer and without modifier): water flux at 0.2 MPa = ca. 2.5 m3/m2-d, membrane II (intermediate compactness prepared from casting solution containing 18% of polymer and without modifier): water flux at 0.2 MPa = ca. 5 m3/m2-d, membrane III (open - prepared from casting solution containing 17% of polymer and without modifier): water flux at 0.2 MPa = ca. 7 m3/m2-d. The dependence of the volumetric water flux (J,) on pressure (AP) for three membranes can be described rather accurately both by the linear function: J, =A,AP and the potential function: J,=A:,
(AP)*
where A, is the permeability of the membrane with respect to water, AL is the regression constant of the potential equation, and b is the exponent of the potential function. Table 4 shows the values of the equation constants obtained from experimental data. The coefficients of correlation and determination resulting from the approximation of the experimental data and the respective equations were very similar to each other. TABLE 4 Values of A,,,, A & and b in equations describing the dependence of volume water flux (J,) on pressure (dP) [J, =A, dPand J,=A& AP*] for tubular membranes made of Winicet (J,., is expressed in m/d and AP in MPa) Membrane type
A,
Dense membrane I Medium porosity membrane II Open membrane III
12.5
A:, 7.00
b 0.622
21.8
10.4
0.507
31.6
20.1
0.691
278
M. Bodzek and K. Konieczny/J. Membrane Sci. 76 (1993) 269-280
TABLE 5 Transport-separation properties of tubular membranes made of Winicet (testing pressure: 0.2 MPa, testing temp.: 298 K, linear velocity over the membrane surface: 3 m/set) Testing substances
Molecular weight
Membrane I Permeate
Membrane II R
flUX
Dextran Dextran Dextran Polyethylene glycol Ovalbumin Albumin y-Globulin EN oil emulsion
40,000 70,000 110,000 15,000 20,000 42,000 69,000 160,000
Permeate
Membrane III R
flux
Permeate
R
flux
(m3/m”-d)
(%)
(m3/m2-d)
(% )
(m3/m2-d)
(%)
1.36 1.52 0.990 1.34 1.27 1.52 1.28 1.24 1.52
82.0 88.0 95.0 22.4 59.1 90.0 92.5 100 92.0’197.02
2.24 2.12 2.08 2.08 2.04 2.12 2.00 1.44 2.08
70.0 81.1 95.0 21.0 51.8 90.0 86.7 100 90.0’/96.8’
2.92 2.43 2.40 3.04 2.88 3.24 2.54 2.48 2.40
52.0 87.0 95.0 20.0 23.0 53.2 87.5 100 80.0’/95.82
‘Retention coefficient of turbidity. 2Retention coefficient of COD. R = retention coefficient.
Table 5 contains the volumetric permeate flux and the experimental retention coefficients obtained for various Winicet membranes differing in their compactness and for various feeds. The volumetric permeate flux decreases with an increase of the molar mass of the permeant and with increasing compactness of the membrane with given macromolecule and emulsion. The most retained substances are particles of oil emulsions in water and proteins. The retention coefficients varied from 100% to 80% depending on membrane compactness and molar mass of the proteins. Dextranes were retained less effectively, though still to a high degree. Dextran with a molar mass of 110,000, for instance, was retained by all the membranes for 95%, whereas the retention coefficient of dextranes with lower molar mass varied within the range 81438% and 52-82%, respectively, depending on the compactness of the membrane. The lowest retention was observed in the case of polyethylene glycols, due to the linear structure of their molecules.
In summary, it may be said that the retention coefficient increases with increasing compactness of the membrane, although the degree of this dependence varies from one test substance to another. Similar dependencies as those mentioned above have been observed earlier in the course of our investigations on polyacrylonitrile-poly (vinyl chloride) membranes [ 1,2] as well as on flat Winicet membranes [lo]. Conclusions (1) Winicet - vinyl chloride-vinyl acetate copolymer - may be used in the production of tubular ultrafiltration membranes by means of the phase-inversion method, applying dimethylformamide as a solvent and water as a precipitating bath. (2) By changing the concentration of the polymer in the casting solution and adding polyethylene glycol to the casting solution membranes of different compactness can be obThe compactness of Winicet tained.
M. Bodzek and K. KoniecznyjJ. Membrane Sci. 76 (1993) 269-280
membranes is also affected by the temperature of gelling of the cast film. (3) The obtained Winicet membranes display an asymmetrical structure, the structure of the membrane sublayer being of the “fingerlike” type with increasing macropore sizes depending on their distance from the skin layer. (4) The skin layer of tubular Winicet membranes is characterized by a narrow range of pore sizes. In the case of membranes obtained from Winicet solutions with a concentration of M-20%, these values range from 50 to 70 nm. (5 ) The nitrogen adsorption/desorption isotherms obtained for tubular Winicet membranes do not display any hysteresis, which proves that no open capillaries are being formed. (6) The surface area of the pores as well as their volume depend on the compactness of the membranes. (7) The dependence of the volumetric water flux (J,) on pressure (AP) is described both by the linear function J,=A,dP and the potential function J, =A L Mb, the permeability of the membrane (A, and A& ) decreasing with increasing compactness and assuming a value of 0.5-0.7 for the potential exponent. (8) Tubular Winicet membranes retain yglobulin as well as colloidal substances completely, and to a high extent also other proteins and dextranes with a molecular mass above 20,000.
References M. Bodzek, Ultrafiltration membranes made of polyacrylonitrile and poly (vinyl chloride ) , in : B. Sedlacek and J. Kahovec (Eds.), Synthetic Polymeric Membranes, W. de Gruyter, Berlin, 1987, pp. 193-202. M. Bodzek and K. Konieczny, The influence of molecular mass of poly (vinyl chloride) on the structure and transport characteristics of ultrafiltration membranes, J. Membrane Sci., 61 (1991) 131. M. Bodzek, K. Konieczny and I. Tanistra, Poly (vinyl chloride) membranes in the treatment of waters and wastewaters, Chemia Stosowana, 32 (1988) 311. M. Bodzek, J. Bohdziewicz, I. Tanistra and K. Kon-
279
ieczny, Application of poly(viny1 chloride) and polyacrylonitrile membranes to concentration, purification and fractionation of macromolecules and colloids, Environ. Protect. Eng., 15 (1989) 121. 5 K. Konieczny and M. Bodzek, Ultrafiltration of water containing colloidal silica, Desalination, 79 (1990) 145. 6 M. Bodzek and K. Konieczny, Polyacrylonitrile and poly(viny1 chloride) ultrafiltration membranes in the treatment of anaphoretic emulsion paint, Desalination, 80 (1991) 43. 7 R.E. Kesting, Synthetic Polymeric Membranes - A Structural Perspective, John Wiley & Sons, New York, NY, 1985. 8 “Chemiplast Erg”, Gliwice, Poland (prospect). 9 Anon., Kunststoff Handbuch Band II, Polyvinylchlorid, Carl Hauser Verlag, Miinchen, 1963. 10 M. Bodzek and K. Konieczny, Ultrafiltration membranes made of Winicet, Polimery, 36 (1991) 233 (in Polish ) . 11 S. Loeb and S. Sourirajan, High-flow semipermeable membranes for separation of water from saline solutions, Adv. Chem. Ser., 38 (1961) 117. 12 W. Kujawski, P. Adamczyk and A. Narcbska, A fully automated system for determination of pore size distribution in microfiltration and ultrafiltration membranes, Sep. Sci. Technol., 24 (1989) 495. 13 J. Broeknoff. Adsorption and capillarity, Ph.D. Thesis, Delft, 1969. 14 M. Bodzek, Determination of pore characteristics of polymer ultrafiltration membranes by gas adsorption/desorption method, Chemia Analityczna, 32 (1985) 563. 15
16
17 18
19
20
21
22 23
R.L. Whistler and J.N. Miller (Eds.), Methods in Carbohydrate Chemistry, Academic Press, New York, NY, 1962. H. Lowry, N.J. Rosenbrough, A.L. Farr and R.J. Randal, Protein measurements with the folin-phenol reagent, J. Biol. Chem., 193 (1951) 261. J.S. Jeris, A rapid COD test, Water Wastes Eng., 4 (1967) 89. H. Strathmann and K. Kock, The formation mechanism of phase-inversion membranes, Desalination, 21 (1977) 241. H. Strathmann and 0. Scheible, Zum Bildungsmechanismus von asymmetrischen Zellulose-acetat Membranen, Kolloid Z.u.Z. Polym., 246 (1971) 669. M. Bodzek, Formation mechanism of asymmetric ultrafiltration membranes, Wiad. Chem., 40 (1986) 101 (in Polish). M. Bodzek, Physico-chemical characterization of ultrafiltration membranes, Polish J. Chem., 57 (1983) 919. S.J. Gregg and K.S. King, Adsorption, Surface Area and Porosity, Izd. Mir, Moskow, 1970. M. Bodzek and J. Bohdziewicz, Porous polycarbonate phase-inversion membranes, J. Membrane Sci., 60 (1991) 25.