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cosalen membrane
Journal of Membrane Science 173 (2000) 99–106
Gas transport properties of HTPB based polyurethane/cosalen membrane Shih-Hsiung Chen a,∗ , Kuang-Chang Yu a , Shih-Liang Houng b , Juin-Yih Lai c a
Department of Environmental Engineering and Health, Chia-Nan College of Pharmacy and Science, Tainan 717, Taiwan, ROC b Department of Chemical Engineering, National Chin-Yi Institute of Technology & Commerce, Taichung, Taiwan, ROC c Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwan, ROC Received 4 August 1999; received in revised form 3 February 2000; accepted 15 February 2000
Abstract Polyurethane (PU) membranes own high gas permeability but low selectivity. In the past, some attempts to increase the gas selectivity by modifying the polymer structure but the effort were unsuccessful. There were other attempts to improve the gas separation by the addition of high affinity salt into a membrane. It was reported that increasing the amount of oxygen carrier salt (cosalen) into a polycarbonate membrane did increase the selectivity of oxygen to nitrogen especially at low temperature [G. Galland, T.M. Lam, J. Appl. Polym. Sci. 50 (1993) 1041]. An increase in salt addition reduced the gas diffusivity but increased the diffusivity ratio of oxygen to nitrogen. In another study, it was also found that the addition of high oxygen-affinity salt into a polycarbonate membrane did not greatly increase the solubility ratio of oxygen to nitrogen but it significantly increased the diffusivity ratio of oxygen to nitrogen [Rouh-Chyu Ruaan, Shih-Hsiung, Chen, Juin-Yih Lai, J. Membr. Sci. 135 (1997) 9]. In this study, the effects of oxygen carrier salt namely, cosalen, in PU membrane on gas separation performance was examined. Gas diffusion and sorption properties were examined by sorption measurements and dual mode analyses. The dual mode analysis showed that the gas separation in PU membrane was dominated by gas diffusion rather than gas sorption. The selectivity of O2 /N2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5◦ C. The key issue of improving the gas separation performance of a polyurethane membrane was to increase the diffusivity ratio but not the solubility ratio. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Polyurethane membrane; Gas separation; Membrane; Oxygen carrier
1. Introduction The most desirable properties of a gas separation membrane are high permeation rate, thermal stability, chemical resistance, and good mechanical properties. Polyurethanes are suitable materials for preparing gas separation membranes with different chemical characteristics and microstructures [1]. However, the use of polyurethane membrane for gas separation is limited by its low selectivity of separation gases. There were ∗
Corresponding author.
a few studies on improving the gas separation performance of PU membranes [2–6]. Most of the past studies focused on the modification chemical structure of PU membrane to improve its gas transport properties. But only few studies could synthesize high performance polyurethane membranes for gas separation. Hsie et al. [3,4] had prepared polyether and polyester base polyurethanes for oxygen and nitrogen separation. It was found that the polyurethane membrane owned high gas permeability but the separation coefficients were mostly lower than 2.0. Chen and Chen [6] had studied the effect of ionization and dispersion on gas transport properties for
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several polyurethane membranes which were synthesized from 4,4-diphenylmethane diisocyanate (MDI), 1,6-hexamethylene diisocyanate (HDI), 2,4-toluene diisocyanate (TDI). It was found that the gas permeation behavior of the PU membrane depended on the glassy transition temperature of the hard domains (Tgh ). The high gas permeation flux membrane could be prepared by controlling the ratio of hard domain to soft domain. However, the gas selectivity could not be improved simply by the change of composition of PU membranes. Pegoraro et al. [2] measured the diffusion coefficient and its dependence on the structure of PUs with bi- and poly-functional polyols and different chain extenders. Gallano and Lam [7] reported that the most important factor in gas diffusion was the molecular weight of soft segment. The effect of chemical composition on the gas permeability might be due to the degree of phase segregation and to the nature of chain packing. In our previous study [8] which used polycarbonate membranes containing an oxygen carrier (cosalen), we found that an increase in the oxygen carrier concentration enhanced the gas selectivity. The purpose of this study was to find out cosalen added to a PU membrane could also change the gas selectivity, permeability, diffusivity and solubility of a PU membrane. Gas sorption measurements were also made to understand the relationship between gas sorption properties and the amount of oxygen carrier in PU membranes.
2. Experimental 2.1. Materials The chemicals used in this study were 4,40 -dicyclohexylmethane diisocyanate (H12 MDI, Desmodur W of Mobay Co.), hydroxyl terminated polybutadiene (HTPB) (equivalent weight 1370 g, R-45 M of ARCO Co.), N-methyl diethanolamine (MDEA) as chain extender and dibutyltin dilaurate (DBTDL) as catalyst. N,N-Dimethyl foramide (DMF) and toluene were used as solvent. The oxygen carrier (cosalen) was prepared by dissolving ethylene diamine, cobalt acetate and salicylaldehyde in ethanol solution, which is refluxed for 2 h. The cosalen crystal formed immediately and was allowed to grow for 2 h out of contact with air. Then cooled and washed with a mount of water. The oxy-
gen carrier was filtrated and dried in a vacuum oven for 24 h at 150◦ C. 2.2. Preparation of polyurethane membrane The two-stage polyurethanes were polymerized first by a –NCO terminated prepolymer and then chain extended with MDEA at 25 wt.% solid content after a theoretical –NCO amount had been reached. Detailed procedures for polymerization had been reported by Huang and Lai [1]. The hard/soft segment ratio of the synthesized PU in terms of moles was 4/1 with the soft segment being 64 wt.% and hard segment 36 wt.%. The PU casting solution was prepared by adding proper amount of HTPB, MDEA and a mixture of DMF and toluene to a reaction vessel. Then a suitable amount of diisocyanate was added for condensation polymerization. The PU membranes were prepared by pouring the solution mixture onto a glass plate to a thickness of 600 m. The PU/cosalen membranes were prepared from PU solution with various amount of oxygen carrier content. When the oxygen carrier content exceeded 5 wt.% in the PU membrane, precipitation of the carrier occurred, there by producing poor membranes. The maximum carrier concentration, therefore, was always 56% in the casting solution. The PU/cosalen membrane was formed with casting the polymer solution onto a glass plate to a predetermined thickness using a Gardner knife at room temperature. Degassing was carried out at 50◦ C for 24 h to evaporate the solvent of casting solution. Dried PU membranes were peeled off from the plate after they had been immersed in deionized water for several hours. The glassy transition temperature of soft and hard segment was −62.6 and 73.8◦ C, respectively. 2.3. Gas permeation measurements Oxygen and nitrogen permeabilities of membranes were determined by using the Yanaco GTR-10 gas permeability analyzer. Detailed procedures for measuring the gas permeation had been reported in a previous publication [9]. The gas permeability was determined by the following equation P =
q/t l (p1 − p2 ) A
S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106
101
where P is the gas permeability (cm3 (STP) cm/cm2 s cm Hg), q/t the volumetric flow rate of gas permeation (cm3 (STP)/s), l the membrane thickness (cm), p1 and p2 are the upstream and downstream pressures (cm Hg), respectively, and A the effective membrane area (cm2 ). The gas separation apparatus must have approached the pressure of about 0.05 Torr by pump in order to remove the residual air for accurate measurements. The gas penetrating through the membrane could be fed into chromatograph by carrier gas to analyze quantity and record by integrator. 2.4. Gas sorption measurements Experimental set-up for gas sorption measurements was shown in our previous report [8]. The amount of gas absorbed was measured by a microbalance. The microbalance (Cahn Model D-202 Electrobalance) was enclosed in a stainless chamber which was placed in a constant temperature box. The system pressure was then vacuumed to about 4×10−3 Torr before the gas sorption measurement and it was kept under this condition until a sorption equilibrium was reached. The gas solubility was calculated by dividing the amount of gas sorption by the corresponding pressure.
3. Results and discussion 3.1. Effect of cosalen content on gas selectivity and permeability The effect of cosalen content on oxygen permeability and selectivity of a PU membrane is shown in Fig. 1. The oxygen permeability decreased with increasing cosalen content in a PU membrane. The oxygen/nitrogen selectivity of cosalen contained membrane increased with increasing cosalen content in the membrane. The original PU membrane exhibited an O2 /N2 selectivity of 2.4. However, the selectivity of the membrane containing 5 wt.% of cosalen reached 4.0 at 35◦ C. The oxygen permeability of the same membrane was 3.1 barrers. It was found that the addition of oxygen carrier into a PU membrane did increase the O2 /N2 selectivity.
Fig. 1. Effect of cosalen content on oxygen permeability and selectivity of oxygen to nitrogen of PU membranes.
3.2. Effect of cosalen content on gas solubility and diffusivity Gas permeation through a membrane can be interpreted as a solution–diffusion process that can be described in terms of solubility and diffusivity for individual polymer and gas [10–12]. Therefore, in order to further understand gas transport properties, the oxygen and nitrogen isotherm were made at 35◦ C. The gas solubility was calculated by dividing the amount of gas sorption by the corresponding pressure. The effect of pressures on gas solubility of PU membrane is shown in Figs. 2 and 3. As can be seen from the figures, oxygen and nitrogen solubilities of the PU membrane increased with increasing cosalen content until a constant value is achieved. Fig. 4 shows the effect of cosalen content on oxygen solubility and the solubility ratio of oxygen to nitrogen at 3 atm. It can be seen that an increase in cosalen content enhanced both oxygen and nitrogen solubility. But the solubility ratio of oxygen to nitrogen did not significantly increase with increasing cosalen content in PU membranes. According to the solution–diffusion mechanism, oxygen selectivity could be contributed by solubility ratio or diffusivity ratio of oxygen to nitrogen. Fig. 4 showed that the solubility ratio remained almost constant, regardless of cosalen content; indicating that the O2 /N2 selectivity was dominated by the
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Fig. 4. Effect of cosalen content on oxygen solubility and the solubility ratio of oxygen to nitrogen at 3 atm. Fig. 2. Effect of pressures on gas solubility of PU membranes. (+) PU; (4) cosalen 1 wt.%; (䉫) cosalen 3 wt.%; (䊊) cosalen: 5 wt.%.
diffusivity ratio of oxygen to nitrogen but not by the solubility ratio. The gas diffusivity of PU membranes was calculated by the relationship of D = P/S, where P is the gas permeability, and S the gas solubility. The effect of cosalen content in the membrane on gas diffusivity
Fig. 3. Effect of pressures on gas solubility of PU membranes. (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen: 5 wt.%.
and diffusivity ratio of oxygen to nitrogen is shown in Fig. 5, which indicated that an increase in the cosalen content in the membrane lowered the oxygen diffusivity. However, the increase in cosalen content raised the diffusivity ratio of oxygen to nitrogen. The above result implied that the addition of cosalen into PU membranes reduced the gas diffusion path and increased the diffusivity ratio. Therefore, it can be concluded that cosalen plays an important role in gas transports through PU membranes.
Fig. 5. Effect of cosalen content on gas diffusivity and diffusivity ratio of oxygen to nitrogen.
S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106
Fig. 6. Effect of operating pressure on gas permeability of PU membranes with various amounts of cosalen at 35◦ C; (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen 5 wt.%.
3.3. Effect of operating pressure on gas separation performance The effect of operating pressure on gas permeability and selectivity of O2 /N2 of PU membranes with various amount of cosalen at 35◦ C is shown in Figs. 6 and 7. The oxygen permeability and the selectivity of oxygen versus nitrogen at various amounts of cosalen contents in PU membranes were almost independent of operating pressure. However, the oxygen permeability of pure a PU membrane slightly decreased with the increase in the operation pressure. An increase in selectivity is evident only at high cosalen percentage in PU membrane. According to the solution–diffusion model, the gas permeability (P) can be described as the product of gas diffusivity (D) and solubility (S) of membranes, namely, P = D × S. The gas diffusivity of a rubbery membrane is almost constant and independent of the operating pressure. As shown in Fig. 6, the gas permeability of a PU membrane significantly decreased with increasing operating pressure. However, the gas permeability of a cosalen/PU membrane only slightly decreased with operating pressure. The gas diffusivity can be obtained by the following equation; namely, D = P/S. The effect of operating pressure on gas diffu-
103
Fig. 7. Effect of operating pressure on gas selectivity of O2 /N2 at various amount of cosalen at 35◦ C; (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen 5 wt.%.
sivity is shown in Figs. 8 and 9. It can be seen that the gas diffusivity of both PU and cosalen contained PU membrane increased with increasing operating pressure; i.e. the gas diffusivity of PU and cosalen
Fig. 8. Effect of operating pressure on oxygen diffusivity of PU membranes, (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen 5 wt.%.
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S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106 Table 1 Dual-mode sorption parameters of PU membranesa Membrane cosalen content (wt.%)
0 1 3 5
Oxygen
Nitrogen
kD
CH0
b
kD
CH0
b
0.08 0.10 0.20 0.25
0.72 0.41 0.30 0.22
1 1 1 1
0.04 0.06 0.18 0.21
0.81 0.27 0.34 0.14
0.5 0.5 0.5 0.5
a Units: k , (cm3 (STP)/cm3 -(polymer)-atm; C 0 , cm3 (STP)/ D H cm3 (polymer); b: 1/atm.
S = SH + SL = kD +
Fig. 9. Effect of operating pressure on nitrogen diffusivity of PU membranes (+) PU;(4) cosalen 1 wt.%; (䊐) cosalen 3 wt.%, (䊊) cosalen 5 wt.%.
contained PU membranes were not constant. Therefore, that the solution–diffusion model can not be used to describe the gas transport behavior of PU membranes. Jordan et al. [13,14] who used the dual sorption and mobility model to describe their results has also reported the dependence of diffusivity on operating pressure. It was proposed that the decrease of gas diffusivity was due to the microstructure change of PU membrane with containing a certain value of cosalen. In order to further understanding the effect of morphology change on gas transport behavior, the gas sorption measurements were made. 3.4. Dual mode analysis The dual-mode sorption parameters were determined by the following expression: C = kd p +
CH0 bp 1 + bp
where kd is Henry’s law coefficient, CH0 and b are saturation capacity and the affinity constant for Langmuir mode, respectively, p gas sorption pressure. The gas solubility was calculated by diving the amount of gas sorption by the corresponding pressure. The gas solubility, therefore, can be described as
CH0 b 1 + bP
where SH and SL are the Henry’s mode solubility and Langmuir’s mode solubility, respectively, kD the Henry’s law dissolution constant, and CH0 and b represent the maximum capacity and equilibrium constant, respectively. The dual-model fitted parameters were obtained from nonlinear-regression of experimental sorption data. The parameters for dual sorption analyses are listed in Table 1. It can be seen that the Henry’s mode solubility increased with increasing cosalen contents in a PU membrane but the Langmuir’s mode solubility decreased. The effect of cosalen content on the ratio of nitrogen Langmuir’s mode solubility (SL ) to Henry’s mode solubility (SH ) is shown in Fig. 10. It was indicated that the ratio of Langmuir’s mode solubility to Henry’s mode solubility decreased with increasing the cosalen content in PU membranes. It was proposed that the Langmuir’s mode solubility be mostly caused by the gas sorption in hard segment region and cosalen in PU membrane. Showing in Fig. 10, the ratio of SL /SH decreased with increasing the cosalen content in PU membranes. It also indicated that the gas solubility ratio of hard segment to soft segment region decreased with increasing cosalen content in a PU membrane. Therefore, the decrease of gas diffusion in the hard segment region was respected while contenting cosalen in PU membranes. It orders to further clarify the decrease in gas diffusivity of cosalen/PU membrane, the analysis of dual mobility were made. The dual mobility parameters are listed in Table 2. In Table 2, DD and DH are mutual diffusion coefficients for the penetrant as defined in the Henry’s
S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106
Fig. 10. Effect of cosalen content on the ratio of nitrogen Langmuir solubility (SL ) to Henry’s solubility (SH ).
law and Langmuir mode, respectively. The diffusion coefficients DD and DH of oxygen and nitrogen were analogous to the diffusion area of a hard segment and soft segment region in PU membranes. It can be seen from in Table 2, both DD and DH decreased with increasing cosalen content. However, both DD and DH decreased but the ratio of DH /DD increased with increasing cosalen content in PU membranes; indicating that the decrease of the gas diffusion in Langmuir type region was less than that in the Henry’s law type region. Therefore, gas diffusion in the hard segment region became more important than that in the soft segment when the cosalen content in the membrane was increased.
105
Fig. 11. Effect of operating temperature on gas permeability of PU membranes. (+) PU; (䉱) cosalen 1 wt.%; (䊉) cosalen 5 wt.%.
3.5. Effect of operating temperature on gas permeability and selectivity The effect of operating temperature on gas permeability and selectivity of PU membranes is shown in Figs. 11 and 12. The oxygen and nitrogen permeability increased with increasing operating tempera-
Table 2 Dual-mobility model parameters of PU membranesa Membrane cosalen Oxygen content (wt.%)
0 1 3 5 a
Nitrogen
DD
DH
DH /DD DD
DH
DH /DD
381.5 150.0 30.0 6.2
6.9 6.1 3.56 1.47
0.02 0.04 0.12 0.24
3.5 2.6 0.9 0.14
0.02 0.05 0.06 0.07
152.2 55.0 14.9 2.0
Units: DD , ×10−8 cm2 s−1 ; DH ,×10−8 cm2 s−1 .
Fig. 12. Effect of operating temperature on gas selectivity of PU membranes. (+) PU; (䉱) cosalen 1 wt.%; (䊉) cosalen 5 wt.%.
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tures. Compared to our previous report, the PU/cosalen membrane shows the same behavior as PC/cosalen membrane at low temperatures. The highest selectivity was obtained at 5◦ C. The selectivity of O2 /N2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5◦ C. 4. Conclusions A PU membrane with the cosalen addition significantly decreased gas permeability but increased oxygen/nitrogen selectivity. The results of sorption and permeation analysis indicated that cosalen lead to a decrease in gas diffusion but an increase in the diffusivity ratio of oxygen to nitrogen. Dual sorption and mobility model analyses indicated that the amount of cosalen in a membrane played an important role in gas diffusion through PU membranes. Gas diffusion in hard segment region became more import than that in soft segment with cosalen contained PU membranes. The highest selectivity was obtained at 5◦ C. The selectivity of O2 /N2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5◦ C. Acknowledgements The authors wish to thank the National Science Council of ROC (NSC 88-2216-E-041-003) for the financial support. References [1] S.L. Huang, J.Y. Lai, Gas permeability of crosslinked HTPB-H12MDI based polyurethane membrane, J. Appl. Polym. Sci. 58 (1995) 1913.
[2] M. Pegoraro, F. Severini, R. Gallo, L. Zanderighi, Gas transport properties of siloxane polyurethanes, J. Appl. Polym. Sci. 57 (1995) 421. [3] K.H. Hsieh, C.C. Tsai, S.M. Tseng, Vapor and gas permeability of polyurethane membranes. Part I. Structure– property relationship, J. Membr. Sci. 49 (1990) 341. [4] K.H. Hsieh, C.C. Tsai, S.M. Tseng, Vapor and gas permeability of polyurethane membranes. Part II. Effect of functional group, J. Membr. Sci. 56 (1991) 279. [5] D.S. Lee, D.S. Jung, T.H. Kim, S.C. Kim, Gas Transport in polyurethane-polystyrene interpenetrating polymer network membranes. I. Effect of synthesis temperature and molecular structure variation, J. Membr. Sci. 60 (1991) 233. [6] W.C. Chen, S.A. Chen, Polyurethane ionomer: effects of emulsification on properties of hexaneethylene diisocyanate based polyether polyurethane cationomers, Polymer 29 (1988) 1995. [7] G. Galland, T.M. Lam, Permeability and diffusion of gases in segmented polyurethanes: structure–property relations, J. Appl. Polym. Sci. 50 (1993) 1041. [8] Shih-Hsiung Chen, Juin-Yih Lai, Polycarbonate/(N, N0 -dialicylidene ethylene diamine) cobalt (II) complexed membrane for gas separation, J. Appl. Polym. Sci. 59 (1996) 1129. [9] Juin-Yih Lai, Shih-Hsiung Chen, Mei-Hsiu Lee, Preparation of polycarbonate/metal salt gas separation membranes, J. Appl. Polym. Sci. 47 (1993) 1513. [10] D.R. Paul, W.J. Koros, Effect of partially immobilizing sorption on permeability and the diffusion time-lag, J. Polym. Sci. Polym. Phys. Ed. 14 (1976) 675. [11] Naoki Toshima, Polymer for Gas Separation, VCH Publishers, New York, 1992. [12] Rouh-Chyu Ruaan, Shih-Hsiung Chen, Juin-Yih Lai, Oxygen/nitrogen separation by polycarbonate/CoSalPr membrane, J. Membr. Sci. 135 (1997) 9. [13] S.M. Jordan, W.J. Koros, G.K. Fleming, The effects of CO2 exposure on pure and mixed gas permeation behavior: comparison of glassy polycarbonate and silicone rubber, J. Membr. Sci. 30 (1987) 191. [14] S.M. Jordan, G.K. Fleming, W.J. Koros, Permeability of carbon dioxide at elevated pressures in substituted polycarbonate, J. Polym. Sci. Polym. Phys. 28 (1990) 2305.
Gas transport properties of HTPB based polyurethane/cosalen membrane Shih-Hsiung Chen a,∗ , Kuang-Chang Yu a , Shih-Liang Houng b , Juin-Yih Lai c a
Department of Environmental Engineering and Health, Chia-Nan College of Pharmacy and Science, Tainan 717, Taiwan, ROC b Department of Chemical Engineering, National Chin-Yi Institute of Technology & Commerce, Taichung, Taiwan, ROC c Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwan, ROC Received 4 August 1999; received in revised form 3 February 2000; accepted 15 February 2000
Abstract Polyurethane (PU) membranes own high gas permeability but low selectivity. In the past, some attempts to increase the gas selectivity by modifying the polymer structure but the effort were unsuccessful. There were other attempts to improve the gas separation by the addition of high affinity salt into a membrane. It was reported that increasing the amount of oxygen carrier salt (cosalen) into a polycarbonate membrane did increase the selectivity of oxygen to nitrogen especially at low temperature [G. Galland, T.M. Lam, J. Appl. Polym. Sci. 50 (1993) 1041]. An increase in salt addition reduced the gas diffusivity but increased the diffusivity ratio of oxygen to nitrogen. In another study, it was also found that the addition of high oxygen-affinity salt into a polycarbonate membrane did not greatly increase the solubility ratio of oxygen to nitrogen but it significantly increased the diffusivity ratio of oxygen to nitrogen [Rouh-Chyu Ruaan, Shih-Hsiung, Chen, Juin-Yih Lai, J. Membr. Sci. 135 (1997) 9]. In this study, the effects of oxygen carrier salt namely, cosalen, in PU membrane on gas separation performance was examined. Gas diffusion and sorption properties were examined by sorption measurements and dual mode analyses. The dual mode analysis showed that the gas separation in PU membrane was dominated by gas diffusion rather than gas sorption. The selectivity of O2 /N2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5◦ C. The key issue of improving the gas separation performance of a polyurethane membrane was to increase the diffusivity ratio but not the solubility ratio. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Polyurethane membrane; Gas separation; Membrane; Oxygen carrier
1. Introduction The most desirable properties of a gas separation membrane are high permeation rate, thermal stability, chemical resistance, and good mechanical properties. Polyurethanes are suitable materials for preparing gas separation membranes with different chemical characteristics and microstructures [1]. However, the use of polyurethane membrane for gas separation is limited by its low selectivity of separation gases. There were ∗
Corresponding author.
a few studies on improving the gas separation performance of PU membranes [2–6]. Most of the past studies focused on the modification chemical structure of PU membrane to improve its gas transport properties. But only few studies could synthesize high performance polyurethane membranes for gas separation. Hsie et al. [3,4] had prepared polyether and polyester base polyurethanes for oxygen and nitrogen separation. It was found that the polyurethane membrane owned high gas permeability but the separation coefficients were mostly lower than 2.0. Chen and Chen [6] had studied the effect of ionization and dispersion on gas transport properties for
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 3 5 7 - 4
100
S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106
several polyurethane membranes which were synthesized from 4,4-diphenylmethane diisocyanate (MDI), 1,6-hexamethylene diisocyanate (HDI), 2,4-toluene diisocyanate (TDI). It was found that the gas permeation behavior of the PU membrane depended on the glassy transition temperature of the hard domains (Tgh ). The high gas permeation flux membrane could be prepared by controlling the ratio of hard domain to soft domain. However, the gas selectivity could not be improved simply by the change of composition of PU membranes. Pegoraro et al. [2] measured the diffusion coefficient and its dependence on the structure of PUs with bi- and poly-functional polyols and different chain extenders. Gallano and Lam [7] reported that the most important factor in gas diffusion was the molecular weight of soft segment. The effect of chemical composition on the gas permeability might be due to the degree of phase segregation and to the nature of chain packing. In our previous study [8] which used polycarbonate membranes containing an oxygen carrier (cosalen), we found that an increase in the oxygen carrier concentration enhanced the gas selectivity. The purpose of this study was to find out cosalen added to a PU membrane could also change the gas selectivity, permeability, diffusivity and solubility of a PU membrane. Gas sorption measurements were also made to understand the relationship between gas sorption properties and the amount of oxygen carrier in PU membranes.
2. Experimental 2.1. Materials The chemicals used in this study were 4,40 -dicyclohexylmethane diisocyanate (H12 MDI, Desmodur W of Mobay Co.), hydroxyl terminated polybutadiene (HTPB) (equivalent weight 1370 g, R-45 M of ARCO Co.), N-methyl diethanolamine (MDEA) as chain extender and dibutyltin dilaurate (DBTDL) as catalyst. N,N-Dimethyl foramide (DMF) and toluene were used as solvent. The oxygen carrier (cosalen) was prepared by dissolving ethylene diamine, cobalt acetate and salicylaldehyde in ethanol solution, which is refluxed for 2 h. The cosalen crystal formed immediately and was allowed to grow for 2 h out of contact with air. Then cooled and washed with a mount of water. The oxy-
gen carrier was filtrated and dried in a vacuum oven for 24 h at 150◦ C. 2.2. Preparation of polyurethane membrane The two-stage polyurethanes were polymerized first by a –NCO terminated prepolymer and then chain extended with MDEA at 25 wt.% solid content after a theoretical –NCO amount had been reached. Detailed procedures for polymerization had been reported by Huang and Lai [1]. The hard/soft segment ratio of the synthesized PU in terms of moles was 4/1 with the soft segment being 64 wt.% and hard segment 36 wt.%. The PU casting solution was prepared by adding proper amount of HTPB, MDEA and a mixture of DMF and toluene to a reaction vessel. Then a suitable amount of diisocyanate was added for condensation polymerization. The PU membranes were prepared by pouring the solution mixture onto a glass plate to a thickness of 600 m. The PU/cosalen membranes were prepared from PU solution with various amount of oxygen carrier content. When the oxygen carrier content exceeded 5 wt.% in the PU membrane, precipitation of the carrier occurred, there by producing poor membranes. The maximum carrier concentration, therefore, was always 56% in the casting solution. The PU/cosalen membrane was formed with casting the polymer solution onto a glass plate to a predetermined thickness using a Gardner knife at room temperature. Degassing was carried out at 50◦ C for 24 h to evaporate the solvent of casting solution. Dried PU membranes were peeled off from the plate after they had been immersed in deionized water for several hours. The glassy transition temperature of soft and hard segment was −62.6 and 73.8◦ C, respectively. 2.3. Gas permeation measurements Oxygen and nitrogen permeabilities of membranes were determined by using the Yanaco GTR-10 gas permeability analyzer. Detailed procedures for measuring the gas permeation had been reported in a previous publication [9]. The gas permeability was determined by the following equation P =
q/t l (p1 − p2 ) A
S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106
101
where P is the gas permeability (cm3 (STP) cm/cm2 s cm Hg), q/t the volumetric flow rate of gas permeation (cm3 (STP)/s), l the membrane thickness (cm), p1 and p2 are the upstream and downstream pressures (cm Hg), respectively, and A the effective membrane area (cm2 ). The gas separation apparatus must have approached the pressure of about 0.05 Torr by pump in order to remove the residual air for accurate measurements. The gas penetrating through the membrane could be fed into chromatograph by carrier gas to analyze quantity and record by integrator. 2.4. Gas sorption measurements Experimental set-up for gas sorption measurements was shown in our previous report [8]. The amount of gas absorbed was measured by a microbalance. The microbalance (Cahn Model D-202 Electrobalance) was enclosed in a stainless chamber which was placed in a constant temperature box. The system pressure was then vacuumed to about 4×10−3 Torr before the gas sorption measurement and it was kept under this condition until a sorption equilibrium was reached. The gas solubility was calculated by dividing the amount of gas sorption by the corresponding pressure.
3. Results and discussion 3.1. Effect of cosalen content on gas selectivity and permeability The effect of cosalen content on oxygen permeability and selectivity of a PU membrane is shown in Fig. 1. The oxygen permeability decreased with increasing cosalen content in a PU membrane. The oxygen/nitrogen selectivity of cosalen contained membrane increased with increasing cosalen content in the membrane. The original PU membrane exhibited an O2 /N2 selectivity of 2.4. However, the selectivity of the membrane containing 5 wt.% of cosalen reached 4.0 at 35◦ C. The oxygen permeability of the same membrane was 3.1 barrers. It was found that the addition of oxygen carrier into a PU membrane did increase the O2 /N2 selectivity.
Fig. 1. Effect of cosalen content on oxygen permeability and selectivity of oxygen to nitrogen of PU membranes.
3.2. Effect of cosalen content on gas solubility and diffusivity Gas permeation through a membrane can be interpreted as a solution–diffusion process that can be described in terms of solubility and diffusivity for individual polymer and gas [10–12]. Therefore, in order to further understand gas transport properties, the oxygen and nitrogen isotherm were made at 35◦ C. The gas solubility was calculated by dividing the amount of gas sorption by the corresponding pressure. The effect of pressures on gas solubility of PU membrane is shown in Figs. 2 and 3. As can be seen from the figures, oxygen and nitrogen solubilities of the PU membrane increased with increasing cosalen content until a constant value is achieved. Fig. 4 shows the effect of cosalen content on oxygen solubility and the solubility ratio of oxygen to nitrogen at 3 atm. It can be seen that an increase in cosalen content enhanced both oxygen and nitrogen solubility. But the solubility ratio of oxygen to nitrogen did not significantly increase with increasing cosalen content in PU membranes. According to the solution–diffusion mechanism, oxygen selectivity could be contributed by solubility ratio or diffusivity ratio of oxygen to nitrogen. Fig. 4 showed that the solubility ratio remained almost constant, regardless of cosalen content; indicating that the O2 /N2 selectivity was dominated by the
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Fig. 4. Effect of cosalen content on oxygen solubility and the solubility ratio of oxygen to nitrogen at 3 atm. Fig. 2. Effect of pressures on gas solubility of PU membranes. (+) PU; (4) cosalen 1 wt.%; (䉫) cosalen 3 wt.%; (䊊) cosalen: 5 wt.%.
diffusivity ratio of oxygen to nitrogen but not by the solubility ratio. The gas diffusivity of PU membranes was calculated by the relationship of D = P/S, where P is the gas permeability, and S the gas solubility. The effect of cosalen content in the membrane on gas diffusivity
Fig. 3. Effect of pressures on gas solubility of PU membranes. (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen: 5 wt.%.
and diffusivity ratio of oxygen to nitrogen is shown in Fig. 5, which indicated that an increase in the cosalen content in the membrane lowered the oxygen diffusivity. However, the increase in cosalen content raised the diffusivity ratio of oxygen to nitrogen. The above result implied that the addition of cosalen into PU membranes reduced the gas diffusion path and increased the diffusivity ratio. Therefore, it can be concluded that cosalen plays an important role in gas transports through PU membranes.
Fig. 5. Effect of cosalen content on gas diffusivity and diffusivity ratio of oxygen to nitrogen.
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Fig. 6. Effect of operating pressure on gas permeability of PU membranes with various amounts of cosalen at 35◦ C; (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen 5 wt.%.
3.3. Effect of operating pressure on gas separation performance The effect of operating pressure on gas permeability and selectivity of O2 /N2 of PU membranes with various amount of cosalen at 35◦ C is shown in Figs. 6 and 7. The oxygen permeability and the selectivity of oxygen versus nitrogen at various amounts of cosalen contents in PU membranes were almost independent of operating pressure. However, the oxygen permeability of pure a PU membrane slightly decreased with the increase in the operation pressure. An increase in selectivity is evident only at high cosalen percentage in PU membrane. According to the solution–diffusion model, the gas permeability (P) can be described as the product of gas diffusivity (D) and solubility (S) of membranes, namely, P = D × S. The gas diffusivity of a rubbery membrane is almost constant and independent of the operating pressure. As shown in Fig. 6, the gas permeability of a PU membrane significantly decreased with increasing operating pressure. However, the gas permeability of a cosalen/PU membrane only slightly decreased with operating pressure. The gas diffusivity can be obtained by the following equation; namely, D = P/S. The effect of operating pressure on gas diffu-
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Fig. 7. Effect of operating pressure on gas selectivity of O2 /N2 at various amount of cosalen at 35◦ C; (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen 5 wt.%.
sivity is shown in Figs. 8 and 9. It can be seen that the gas diffusivity of both PU and cosalen contained PU membrane increased with increasing operating pressure; i.e. the gas diffusivity of PU and cosalen
Fig. 8. Effect of operating pressure on oxygen diffusivity of PU membranes, (+) PU; (䉱) cosalen 1 wt.%; (䊏) cosalen 3 wt.%; (䊉) cosalen 5 wt.%.
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S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106 Table 1 Dual-mode sorption parameters of PU membranesa Membrane cosalen content (wt.%)
0 1 3 5
Oxygen
Nitrogen
kD
CH0
b
kD
CH0
b
0.08 0.10 0.20 0.25
0.72 0.41 0.30 0.22
1 1 1 1
0.04 0.06 0.18 0.21
0.81 0.27 0.34 0.14
0.5 0.5 0.5 0.5
a Units: k , (cm3 (STP)/cm3 -(polymer)-atm; C 0 , cm3 (STP)/ D H cm3 (polymer); b: 1/atm.
S = SH + SL = kD +
Fig. 9. Effect of operating pressure on nitrogen diffusivity of PU membranes (+) PU;(4) cosalen 1 wt.%; (䊐) cosalen 3 wt.%, (䊊) cosalen 5 wt.%.
contained PU membranes were not constant. Therefore, that the solution–diffusion model can not be used to describe the gas transport behavior of PU membranes. Jordan et al. [13,14] who used the dual sorption and mobility model to describe their results has also reported the dependence of diffusivity on operating pressure. It was proposed that the decrease of gas diffusivity was due to the microstructure change of PU membrane with containing a certain value of cosalen. In order to further understanding the effect of morphology change on gas transport behavior, the gas sorption measurements were made. 3.4. Dual mode analysis The dual-mode sorption parameters were determined by the following expression: C = kd p +
CH0 bp 1 + bp
where kd is Henry’s law coefficient, CH0 and b are saturation capacity and the affinity constant for Langmuir mode, respectively, p gas sorption pressure. The gas solubility was calculated by diving the amount of gas sorption by the corresponding pressure. The gas solubility, therefore, can be described as
CH0 b 1 + bP
where SH and SL are the Henry’s mode solubility and Langmuir’s mode solubility, respectively, kD the Henry’s law dissolution constant, and CH0 and b represent the maximum capacity and equilibrium constant, respectively. The dual-model fitted parameters were obtained from nonlinear-regression of experimental sorption data. The parameters for dual sorption analyses are listed in Table 1. It can be seen that the Henry’s mode solubility increased with increasing cosalen contents in a PU membrane but the Langmuir’s mode solubility decreased. The effect of cosalen content on the ratio of nitrogen Langmuir’s mode solubility (SL ) to Henry’s mode solubility (SH ) is shown in Fig. 10. It was indicated that the ratio of Langmuir’s mode solubility to Henry’s mode solubility decreased with increasing the cosalen content in PU membranes. It was proposed that the Langmuir’s mode solubility be mostly caused by the gas sorption in hard segment region and cosalen in PU membrane. Showing in Fig. 10, the ratio of SL /SH decreased with increasing the cosalen content in PU membranes. It also indicated that the gas solubility ratio of hard segment to soft segment region decreased with increasing cosalen content in a PU membrane. Therefore, the decrease of gas diffusion in the hard segment region was respected while contenting cosalen in PU membranes. It orders to further clarify the decrease in gas diffusivity of cosalen/PU membrane, the analysis of dual mobility were made. The dual mobility parameters are listed in Table 2. In Table 2, DD and DH are mutual diffusion coefficients for the penetrant as defined in the Henry’s
S.-H. Chen et al. / Journal of Membrane Science 173 (2000) 99–106
Fig. 10. Effect of cosalen content on the ratio of nitrogen Langmuir solubility (SL ) to Henry’s solubility (SH ).
law and Langmuir mode, respectively. The diffusion coefficients DD and DH of oxygen and nitrogen were analogous to the diffusion area of a hard segment and soft segment region in PU membranes. It can be seen from in Table 2, both DD and DH decreased with increasing cosalen content. However, both DD and DH decreased but the ratio of DH /DD increased with increasing cosalen content in PU membranes; indicating that the decrease of the gas diffusion in Langmuir type region was less than that in the Henry’s law type region. Therefore, gas diffusion in the hard segment region became more important than that in the soft segment when the cosalen content in the membrane was increased.
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Fig. 11. Effect of operating temperature on gas permeability of PU membranes. (+) PU; (䉱) cosalen 1 wt.%; (䊉) cosalen 5 wt.%.
3.5. Effect of operating temperature on gas permeability and selectivity The effect of operating temperature on gas permeability and selectivity of PU membranes is shown in Figs. 11 and 12. The oxygen and nitrogen permeability increased with increasing operating tempera-
Table 2 Dual-mobility model parameters of PU membranesa Membrane cosalen Oxygen content (wt.%)
0 1 3 5 a
Nitrogen
DD
DH
DH /DD DD
DH
DH /DD
381.5 150.0 30.0 6.2
6.9 6.1 3.56 1.47
0.02 0.04 0.12 0.24
3.5 2.6 0.9 0.14
0.02 0.05 0.06 0.07
152.2 55.0 14.9 2.0
Units: DD , ×10−8 cm2 s−1 ; DH ,×10−8 cm2 s−1 .
Fig. 12. Effect of operating temperature on gas selectivity of PU membranes. (+) PU; (䉱) cosalen 1 wt.%; (䊉) cosalen 5 wt.%.
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tures. Compared to our previous report, the PU/cosalen membrane shows the same behavior as PC/cosalen membrane at low temperatures. The highest selectivity was obtained at 5◦ C. The selectivity of O2 /N2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5◦ C. 4. Conclusions A PU membrane with the cosalen addition significantly decreased gas permeability but increased oxygen/nitrogen selectivity. The results of sorption and permeation analysis indicated that cosalen lead to a decrease in gas diffusion but an increase in the diffusivity ratio of oxygen to nitrogen. Dual sorption and mobility model analyses indicated that the amount of cosalen in a membrane played an important role in gas diffusion through PU membranes. Gas diffusion in hard segment region became more import than that in soft segment with cosalen contained PU membranes. The highest selectivity was obtained at 5◦ C. The selectivity of O2 /N2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5◦ C. Acknowledgements The authors wish to thank the National Science Council of ROC (NSC 88-2216-E-041-003) for the financial support. References [1] S.L. Huang, J.Y. Lai, Gas permeability of crosslinked HTPB-H12MDI based polyurethane membrane, J. Appl. Polym. Sci. 58 (1995) 1913.
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