- Email: [email protected]
Effect of Diluent on the Morphology and Performance of IPP Hollow Fiber Microporous Membrane via Thermally Induced Phase Separation1
Chinese J. Chem. Eng., 14(3) 394-397
(2006)
RESEARCH NOTES
Effect of Diluent on the Morphology and Performance of IPP Hollow Fiber Microporous Membrane via Thermally Induced Phase Separation* YANG Zhensheng(&&&)a*b,LI Pingli(* Shichang(3t& B )" a
%A)"***,CHANG Heying('#$ % $$)a
and WANG
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China
Abstract Isotactic polypropylene (iPP) hollow fiber microporous membranes were prepared using thermally induced phase separation (TIPS) method. Di-n-butyl phthalate (DBP), dioctyl phthalate (DOP), and the mixed solvent were used as diluents. The effect of a (DOP mass fraction in diluent) on the morphology and performance of the hollow fiber was investigated. With increasing a, the morphology of the resulting hollow fiber changes from typical cellular structure to mixed structure, and then to typical particulate structure. As a result, the permeability of the hollow fiber increases sharply, and the mechanical properties of the hollow fiber decrease obviously. It is suggested that the morphology and performances of iPP hollow fiber microporous membrane can be controlled via adjusting the compatibility between iPP and diluent. Keywords thermally induced phase separation, hollow fiber, isotactic polypropylene, membrane, morphology
1 INTRODUCTION Isotactic polypropylene (iPP) is an outstanding membrane material in terms of low cost and good resistance to chemicals. However, solvent-casting iPP into membrane at low temperature is a challenge due to the lack of room-temperature solvent of iPP. The thermally induced phase separation (TIPS), based on the dependence of the polymer solubility on the temperature, offers an attractive way to prepare iPP membrane because iPP can be dissolved in some solvents at about melting temperature of pure iPP"]. Normally, the selection of dilution plays a key role in TIPS membrane preparation. Thermodynamic interaction between the polymer and diluent, that is, the compatibility between the polymer and diluent, does not only affect the phase diagram"-31 but also As a result, the morthe kinetics of pore phology and performance of the membrane can be controlled['41. Besides, by evaporating the low boiling point diluent from the top surface of the membrane, anisotropic or asymmetric membrane can be formed, Therefore, the performance of the membrane can be easily changedF5'. However, no literature is found on the effect of diluent on iPP hollow fiber membrane via the TIPS process.
Prior to this investigation, the thermally induced phase separation thermodynamics of the ternary solution consisting of iPP, di-n-butyl phthalate (DBP) and dioctyl phthalate (DOP) was studied by pseudo-binary approach[61.The aim of this work is to investigate the effect of diluents on the morphology and performance of iPP hollow fiber microporous membrane further. The DBP, DOP, and the mixed solvent were used as diluents, respectively. It is noted that the evaporation of the diluent was not considered because the boiling points of DBP and DOP are 612K and 659K at atmosphere pressure[71 respectively, and the spinning temperature of the hollow fiber is 413K.
2 EXPERIMENTAL 2.1 Materials IPP (melt index=3.0g.(lOmin) average molecular weight=412OOO) was purchased from Daqing Petroleum Chemical (China). DBP (analysis grade) and DOP (analysis grade) were obtained from Tianjin Bodi Chemical Reagents (China), the content of DBP and DOP is not less than 99.5% and 99.0%, respectively. All materials were used without further purification.
Received 2005-08-18, accepted 2006-02-24.
* Supported by the National Natural Science Foundation of China (No.20236030). ** To whom correspondenceshould be addressed. E-mail: [email protected]
-',
Effect of Diluent on the Morphology and Performance of IPP Hollow Fiber Microporous Membrane via TIPS
2.2 Prepmalion of hollow fiber membrane The polymer mass fraction was fixed at 0.45 in the spinning of the hollow fiber. Certain amounts of iPP, DBP and DOP were added to a stainless steel vessel. The mixed iPP/diluent system had been heated and stirred about 6h at 473K under the nitrogen gas atmosphere until a homogeneous solution was obtained. The solution was degassed under vacuum to remove gas bubbles before being fed to a tube-in-orifice (inner diameter: 8.2mm, outer diameter: 9 . 6 ~ spinneret. ) Nitrogen gas was used as bore fluid at a rate of 120mlmin-'. Hollow fiber was extruded from the spinneret at a rate of about 4.25ml.min-' by nitrogen pressure of 0.3MPa, moved through the air quencher to induce the phase separation and solidification, and then wound on the take-up winder at 1 0 . 2 m m 6 ' . The temperature of spinneret and air quencher was kept at 413K and 283K3,respectively. The diluent was extracted from the hollow fiber by immersing it into isopropyl alcohol. The outer diameter and the wall thickness of the resulting hollow fibers were (1050 3 0 ) p , (120 k 1 O ) p , respectively.
nate from the mathematical model for iPP/DBP/DOP systems[61.For a (DOP mass fraction in diluent)=O, 0.50, 0.65, 0.80, with increasing a, binodal curves shift to lower temperature region because the compatibility between iPP and co-solvent becomes higher with increasing a. In the case of a=1, only the dynamic crystallization line can be observed while binodal curve is not because the compatibility between iPP and DOP is quite better. The dynamic crystallization line is hardly influenced by varying a. It can be shown that the dynamic crystallization line shifts obviously to lower temperature region with increasing cooling rate at a=l.
I
0.2
0
2.3 Characterization of the hollow fiber membrane The images of the hollow fibers were observed by a PHILIPS XL30 scanning electron microscope (SEM). The pure water flux of single fiber was measured at O.lMPa, similar to that described by Matsuyama et al.[*].Pure water was forced to permeate from the inside to the outside of the hollow fiber. The pure water flux was calculated on the basis of the inner surface area of the hollow fiber. The tensile strength and breaking elongation of a single hollow fiber were measured by WD-1OD (Changchun Second Testing Machines, China). Gage length and X-head speed were 40mm and 50mmmin-', respectively. 3 RESULTS AND DISCUSSION 3.1 Pseudo-binaryphase diagrams Both measurement and correlation of the phase diagrams for iPP/DBP/DOP systems were demonstrated in our previous study[61.A brief introduction is as below. The pseudo-binary phase diagrams for the iPP/diluent systems are shown in Fig.1, in which the symbols are experimental results and the curves origi-
395
0.4 0.6 0.8 polymer volume fraction
1.o
Figure 1 Pseudo-binary phase diagrams for iPP/diluent systems 0 A r-liquid-liquid phase separation temperature; 0 0A V x -dynamic crystallization temperature; u (cooling rate=5Kmin-'): W 0 0; 0 0 0.50; A A 0.65; V V 0.80; cooling rate, Kmin-': (u=l) 0 5; x 20
*
3.2 Membrane morphology The effect of diluent on membrane morphology is illustrated in Fig.2. When a=O (the diluent is DBP), the hollow fiber shows a typically cellular structure related to liquid-liquid phase separation because the liquid-liquid phase separation temperature (Tcloudd25.1Kfrom Fig.1) is much higher than the dynamic crystallization temperature (Tcy =379.5K, based on iPP/DOP system at 20K.min-' from Fig.1). When a=l (the diluent is DOP), the hollow fiber exhibits a typical spherulitic particulttit: structure originating from polymer crystallization. When a=0.50, the hollow fiber exhibits a mixed structure, which is basically cellular but with particulate boundaries, because liquid-liquid phase separation happened prior to polymer crystallization according to TCl,,d=406.25K and Tcy=379.5K.When a a . 6 5 , it is similar to ad.50. Chinese J. Ch. E. 14(3) 394 (2006)
Chinese J. Ch. E. (Vol. 14, No3)
3%
At a =0.80,polymer crystallization happens with liquid-liquid phase separation nearly at the same time because Tcloud (380.15K)is near to Tcy (379.5K). Therefore, the hollow fiber shows an incomplete crystalline particulate structure which is an intermediate stages of the spherulite growth"].
the morphology of the hollow fiber changes from typical cellular structure to mixed structure, and then particulate structure. When the morphology of the hollow fiber is a mixed structure, plentiful voids in the cellular walls connect the cells into an open cellular structure. When the morphology of the hollow fiber is a particulate structure, voids between the aggregated particles form a continuous and interconnected porous network. 400 -
(e) a=l
It is indicated that the connectivity between the pores and the water permeability of the resulting hollow fiber can be significantly improved by increasing the relative weight of particulate structure.
Figure 2 SEM images of WP hollow fibers (cross section near to inner side of hollow fiber, 3000 X )
It is proposed that any membrane morphology in this investigation is regarded as a mixture of typical particulate structure and typical cellular one. The relative weight of particulate structure increases while that of cellular one decreases with increasing a. It is revealed that the membrane morphology can be controlled by adjusting the compatibility between iPP and diluent.
3.3 Pure water flux The effect of diluent on pure water flux of the hollow fiber is shown in Fig.3. Here, it is noted that when a=1, the mechanical strength of the resulting hollow fiber is so weak that the pure water flux can not be measured. The pure water flux of the hollow fiber increases with increasing a. At a=O, the pure w&er flux of the hollow fiber is very low because typical cellular pores are largely isolated and embedded in a continuous polymer matrix. When a increases, June, 2006
3.4 Mechanical properties The effect of diluent on the mechanical properties of the hollow fiber is shown in Figs.4 and 5. The tensile strength and breaking elongation of the hollow fiber decrease with increasing a. Both figures reveal that the mechanical properties of the hollow fiber with typical cellular structure or mixed membrane structure are higher than those of the hollow fiber with particulate structure.
DOP mass fraction in diluent
Figure 4 Effect of diluent on tensile strength
Effect of Diluent on the Morphology and Performance of EPP Hollow Fiber Microporous Membrane via TIPS
s
*O0
h I
\
I
4
\
\.
I
0
0.2
I
--L7
0.4 0.6 0.8 DOP mass fraction in diluent
1.o
5
Figure 5 Effect of diluent on breaking elongation
It is concluded that the permeability and mechanical properties of the membrane show contrary trend when varying diluent. Therefore, the iPP hollow fiber microporous membrane with higher permeability and higher mechanical properties must exhibit a mixed membrane morphology which is basically cellular but with particulate boundaries, in which the liquid-liquid phase separation precedes polymer crystallization.
6
7
8
9
REFERENCES 1
Lloyd, D.R., Kim, S.S., Kinzer, K.E., “Microporous membrane formation via thermally-induced phase separation. II. Liquid-liquid phase Separation”,J. Membr. Sci., 64, 1-ll(1991).
397
Matsuyama, H., Teramoto, M., Kudari, S., “Effect of diluents on membrane formation via thermally induced phase separation”,J. Appl. Polym. Sci., 82, 169-177(2001). Vadalia, H.C., Lee, H.K., Myerson, AS., Levon, K., “Thermally induced phase separation in ternary crystallizable polymer solutions”, J. Membr. Sci., 89, 37SO( 1994). Mcguire, K.S., Laxminarayan, A., Lloyd, D.R., “Kinetics of droplet growth in liquid-liquid phase separation of polymer-diluent systems: Experimental results”, Polymer, 36,495 1-496O( 1995). Hellman, D.J., Greenberg, A.R., Krantz, W.B., “A novel process for membrane fabrication: Thermally assisted evaporative phase separation (TAEPS)”, J. Membr. Sci., 230,99-109(2004). Yang, Z.S., Chang, H.Y., Li, P.L., Wang, S.C., “Thermodynamics of thermally induced phase separation of iPP-DBP-DOP ternary solution by pseudo-binary approach’’, J. Chem. Ind. Eng. (China), 56(6), 981988(2005). (in Chinese) Brandrup, J., Immergut, E.H., Grulke, E.A., Polymer Handbook, 4th edition, John Wiley, New York (1999). Matsuyama, H., Okafuji, H., Maki, T., Teramoto, M., Kubota, N., “Preparation of polyethylene hollow fiber membrane via thermally induced phase separation”, J . Membr. Sci., 223, 11F126(2003). Young ,T.H., Lin, D.J., Gau, J.J., Chuang, W.Y., Cheng, L.P., “Morphology of crystalline Nylon-610 membranes prepared by the immersion-precipitation process: Competition between crystallization and liquid-liquid phase separation”, Polymer, 40,501 1-5021( 1999).
Chinese J. Ch. E. 14(3) 394 (2006)
(2006)
RESEARCH NOTES
Effect of Diluent on the Morphology and Performance of IPP Hollow Fiber Microporous Membrane via Thermally Induced Phase Separation* YANG Zhensheng(&&&)a*b,LI Pingli(* Shichang(3t& B )" a
%A)"***,CHANG Heying('#$ % $$)a
and WANG
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China
Abstract Isotactic polypropylene (iPP) hollow fiber microporous membranes were prepared using thermally induced phase separation (TIPS) method. Di-n-butyl phthalate (DBP), dioctyl phthalate (DOP), and the mixed solvent were used as diluents. The effect of a (DOP mass fraction in diluent) on the morphology and performance of the hollow fiber was investigated. With increasing a, the morphology of the resulting hollow fiber changes from typical cellular structure to mixed structure, and then to typical particulate structure. As a result, the permeability of the hollow fiber increases sharply, and the mechanical properties of the hollow fiber decrease obviously. It is suggested that the morphology and performances of iPP hollow fiber microporous membrane can be controlled via adjusting the compatibility between iPP and diluent. Keywords thermally induced phase separation, hollow fiber, isotactic polypropylene, membrane, morphology
1 INTRODUCTION Isotactic polypropylene (iPP) is an outstanding membrane material in terms of low cost and good resistance to chemicals. However, solvent-casting iPP into membrane at low temperature is a challenge due to the lack of room-temperature solvent of iPP. The thermally induced phase separation (TIPS), based on the dependence of the polymer solubility on the temperature, offers an attractive way to prepare iPP membrane because iPP can be dissolved in some solvents at about melting temperature of pure iPP"]. Normally, the selection of dilution plays a key role in TIPS membrane preparation. Thermodynamic interaction between the polymer and diluent, that is, the compatibility between the polymer and diluent, does not only affect the phase diagram"-31 but also As a result, the morthe kinetics of pore phology and performance of the membrane can be controlled['41. Besides, by evaporating the low boiling point diluent from the top surface of the membrane, anisotropic or asymmetric membrane can be formed, Therefore, the performance of the membrane can be easily changedF5'. However, no literature is found on the effect of diluent on iPP hollow fiber membrane via the TIPS process.
Prior to this investigation, the thermally induced phase separation thermodynamics of the ternary solution consisting of iPP, di-n-butyl phthalate (DBP) and dioctyl phthalate (DOP) was studied by pseudo-binary approach[61.The aim of this work is to investigate the effect of diluents on the morphology and performance of iPP hollow fiber microporous membrane further. The DBP, DOP, and the mixed solvent were used as diluents, respectively. It is noted that the evaporation of the diluent was not considered because the boiling points of DBP and DOP are 612K and 659K at atmosphere pressure[71 respectively, and the spinning temperature of the hollow fiber is 413K.
2 EXPERIMENTAL 2.1 Materials IPP (melt index=3.0g.(lOmin) average molecular weight=412OOO) was purchased from Daqing Petroleum Chemical (China). DBP (analysis grade) and DOP (analysis grade) were obtained from Tianjin Bodi Chemical Reagents (China), the content of DBP and DOP is not less than 99.5% and 99.0%, respectively. All materials were used without further purification.
Received 2005-08-18, accepted 2006-02-24.
* Supported by the National Natural Science Foundation of China (No.20236030). ** To whom correspondenceshould be addressed. E-mail: [email protected]
-',
Effect of Diluent on the Morphology and Performance of IPP Hollow Fiber Microporous Membrane via TIPS
2.2 Prepmalion of hollow fiber membrane The polymer mass fraction was fixed at 0.45 in the spinning of the hollow fiber. Certain amounts of iPP, DBP and DOP were added to a stainless steel vessel. The mixed iPP/diluent system had been heated and stirred about 6h at 473K under the nitrogen gas atmosphere until a homogeneous solution was obtained. The solution was degassed under vacuum to remove gas bubbles before being fed to a tube-in-orifice (inner diameter: 8.2mm, outer diameter: 9 . 6 ~ spinneret. ) Nitrogen gas was used as bore fluid at a rate of 120mlmin-'. Hollow fiber was extruded from the spinneret at a rate of about 4.25ml.min-' by nitrogen pressure of 0.3MPa, moved through the air quencher to induce the phase separation and solidification, and then wound on the take-up winder at 1 0 . 2 m m 6 ' . The temperature of spinneret and air quencher was kept at 413K and 283K3,respectively. The diluent was extracted from the hollow fiber by immersing it into isopropyl alcohol. The outer diameter and the wall thickness of the resulting hollow fibers were (1050 3 0 ) p , (120 k 1 O ) p , respectively.
nate from the mathematical model for iPP/DBP/DOP systems[61.For a (DOP mass fraction in diluent)=O, 0.50, 0.65, 0.80, with increasing a, binodal curves shift to lower temperature region because the compatibility between iPP and co-solvent becomes higher with increasing a. In the case of a=1, only the dynamic crystallization line can be observed while binodal curve is not because the compatibility between iPP and DOP is quite better. The dynamic crystallization line is hardly influenced by varying a. It can be shown that the dynamic crystallization line shifts obviously to lower temperature region with increasing cooling rate at a=l.
I
0.2
0
2.3 Characterization of the hollow fiber membrane The images of the hollow fibers were observed by a PHILIPS XL30 scanning electron microscope (SEM). The pure water flux of single fiber was measured at O.lMPa, similar to that described by Matsuyama et al.[*].Pure water was forced to permeate from the inside to the outside of the hollow fiber. The pure water flux was calculated on the basis of the inner surface area of the hollow fiber. The tensile strength and breaking elongation of a single hollow fiber were measured by WD-1OD (Changchun Second Testing Machines, China). Gage length and X-head speed were 40mm and 50mmmin-', respectively. 3 RESULTS AND DISCUSSION 3.1 Pseudo-binaryphase diagrams Both measurement and correlation of the phase diagrams for iPP/DBP/DOP systems were demonstrated in our previous study[61.A brief introduction is as below. The pseudo-binary phase diagrams for the iPP/diluent systems are shown in Fig.1, in which the symbols are experimental results and the curves origi-
395
0.4 0.6 0.8 polymer volume fraction
1.o
Figure 1 Pseudo-binary phase diagrams for iPP/diluent systems 0 A r-liquid-liquid phase separation temperature; 0 0A V x -dynamic crystallization temperature; u (cooling rate=5Kmin-'): W 0 0; 0 0 0.50; A A 0.65; V V 0.80; cooling rate, Kmin-': (u=l) 0 5; x 20
*
3.2 Membrane morphology The effect of diluent on membrane morphology is illustrated in Fig.2. When a=O (the diluent is DBP), the hollow fiber shows a typically cellular structure related to liquid-liquid phase separation because the liquid-liquid phase separation temperature (Tcloudd25.1Kfrom Fig.1) is much higher than the dynamic crystallization temperature (Tcy =379.5K, based on iPP/DOP system at 20K.min-' from Fig.1). When a=l (the diluent is DOP), the hollow fiber exhibits a typical spherulitic particulttit: structure originating from polymer crystallization. When a=0.50, the hollow fiber exhibits a mixed structure, which is basically cellular but with particulate boundaries, because liquid-liquid phase separation happened prior to polymer crystallization according to TCl,,d=406.25K and Tcy=379.5K.When a a . 6 5 , it is similar to ad.50. Chinese J. Ch. E. 14(3) 394 (2006)
Chinese J. Ch. E. (Vol. 14, No3)
3%
At a =0.80,polymer crystallization happens with liquid-liquid phase separation nearly at the same time because Tcloud (380.15K)is near to Tcy (379.5K). Therefore, the hollow fiber shows an incomplete crystalline particulate structure which is an intermediate stages of the spherulite growth"].
the morphology of the hollow fiber changes from typical cellular structure to mixed structure, and then particulate structure. When the morphology of the hollow fiber is a mixed structure, plentiful voids in the cellular walls connect the cells into an open cellular structure. When the morphology of the hollow fiber is a particulate structure, voids between the aggregated particles form a continuous and interconnected porous network. 400 -
(e) a=l
It is indicated that the connectivity between the pores and the water permeability of the resulting hollow fiber can be significantly improved by increasing the relative weight of particulate structure.
Figure 2 SEM images of WP hollow fibers (cross section near to inner side of hollow fiber, 3000 X )
It is proposed that any membrane morphology in this investigation is regarded as a mixture of typical particulate structure and typical cellular one. The relative weight of particulate structure increases while that of cellular one decreases with increasing a. It is revealed that the membrane morphology can be controlled by adjusting the compatibility between iPP and diluent.
3.3 Pure water flux The effect of diluent on pure water flux of the hollow fiber is shown in Fig.3. Here, it is noted that when a=1, the mechanical strength of the resulting hollow fiber is so weak that the pure water flux can not be measured. The pure water flux of the hollow fiber increases with increasing a. At a=O, the pure w&er flux of the hollow fiber is very low because typical cellular pores are largely isolated and embedded in a continuous polymer matrix. When a increases, June, 2006
3.4 Mechanical properties The effect of diluent on the mechanical properties of the hollow fiber is shown in Figs.4 and 5. The tensile strength and breaking elongation of the hollow fiber decrease with increasing a. Both figures reveal that the mechanical properties of the hollow fiber with typical cellular structure or mixed membrane structure are higher than those of the hollow fiber with particulate structure.
DOP mass fraction in diluent
Figure 4 Effect of diluent on tensile strength
Effect of Diluent on the Morphology and Performance of EPP Hollow Fiber Microporous Membrane via TIPS
s
*O0
h I
\
I
4
\
\.
I
0
0.2
I
--L7
0.4 0.6 0.8 DOP mass fraction in diluent
1.o
5
Figure 5 Effect of diluent on breaking elongation
It is concluded that the permeability and mechanical properties of the membrane show contrary trend when varying diluent. Therefore, the iPP hollow fiber microporous membrane with higher permeability and higher mechanical properties must exhibit a mixed membrane morphology which is basically cellular but with particulate boundaries, in which the liquid-liquid phase separation precedes polymer crystallization.
6
7
8
9
REFERENCES 1
Lloyd, D.R., Kim, S.S., Kinzer, K.E., “Microporous membrane formation via thermally-induced phase separation. II. Liquid-liquid phase Separation”,J. Membr. Sci., 64, 1-ll(1991).
397
Matsuyama, H., Teramoto, M., Kudari, S., “Effect of diluents on membrane formation via thermally induced phase separation”,J. Appl. Polym. Sci., 82, 169-177(2001). Vadalia, H.C., Lee, H.K., Myerson, AS., Levon, K., “Thermally induced phase separation in ternary crystallizable polymer solutions”, J. Membr. Sci., 89, 37SO( 1994). Mcguire, K.S., Laxminarayan, A., Lloyd, D.R., “Kinetics of droplet growth in liquid-liquid phase separation of polymer-diluent systems: Experimental results”, Polymer, 36,495 1-496O( 1995). Hellman, D.J., Greenberg, A.R., Krantz, W.B., “A novel process for membrane fabrication: Thermally assisted evaporative phase separation (TAEPS)”, J. Membr. Sci., 230,99-109(2004). Yang, Z.S., Chang, H.Y., Li, P.L., Wang, S.C., “Thermodynamics of thermally induced phase separation of iPP-DBP-DOP ternary solution by pseudo-binary approach’’, J. Chem. Ind. Eng. (China), 56(6), 981988(2005). (in Chinese) Brandrup, J., Immergut, E.H., Grulke, E.A., Polymer Handbook, 4th edition, John Wiley, New York (1999). Matsuyama, H., Okafuji, H., Maki, T., Teramoto, M., Kubota, N., “Preparation of polyethylene hollow fiber membrane via thermally induced phase separation”, J . Membr. Sci., 223, 11F126(2003). Young ,T.H., Lin, D.J., Gau, J.J., Chuang, W.Y., Cheng, L.P., “Morphology of crystalline Nylon-610 membranes prepared by the immersion-precipitation process: Competition between crystallization and liquid-liquid phase separation”, Polymer, 40,501 1-5021( 1999).
Chinese J. Ch. E. 14(3) 394 (2006)