- Email: [email protected]
Towards single step production of multi-layer inorganic hollow fibers
Journal of Membrane Science 239 (2004) 265–269
Towards single step production of multi-layer inorganic hollow fibers J. de Jong, N.E. Benes1 , G.H. Koops, M. Wessling∗ Membrane Technology Group, Faculty of Science and Technology, University of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands Received 1 September 2003; received in revised form 5 December 2003; accepted 2 February 2004
Abstract In this work we propose a generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning combined with a heat treatment. With this new method, membranes with a high surface area per unit volume ratio can be produced, while production time and costs are dramatically reduced. The proof-of-principle of the concept will be demonstrated with the production of double layer ␣-alumina hollow fibers. Although various problems were anticipated at the interface of the layers, the adhesion between the two layers is surprisingly good, both in the precursor and the sintered fiber. Produced fibers show an asymmetric structure with a porosity between 37 and 45%. The macrostructure of the sintered fiber is largely determined by the macrostructure of the precursor fiber, while differences in microstructure disappear during the heat treatment step. The proposed method is not limited to ␣-alumina membranes; in principle many ceramic or metallic powders may be used. This means that this method can open up the way for a new generation of membranes. © 2004 Elsevier B.V. All rights reserved. Keywords: Wet spinning; Co-extrusion; Co-sintering; Ceramic double layer hollow fiber; Membranes
1. Introduction Because of their thermal and chemical stability, inorganic membranes offer an attractive alternative for separation processes where organic polymer membranes cannot be used. However, inorganic membranes show some major drawbacks, including: (a) high price; (b) long and complicated production process; and (c) low surface area per unit volume (A/V ratio). The high price is not merely due to the starting materials, but can be attributed to the complicated, time and energy consuming production process, which generally comprises several sequential steps. First a support layer is made to provide mechanical strength to the membrane. Subsequently, on top of this support one or more intermediate layers have to be coated, before the final separation layer can be applied. Each step includes an expensive heat treatment. Clearly, a reduction or combination of steps is desired to cut production time and costs and thereby membrane price. The price could be decreased even further by increasing the A/V ratio of the membranes. Membranes with a hollow fiber geometry are in this respect ∗
Corresponding author. Tel.: +31 53 489 2951; fax: +31 53 489 4611. E-mail address: [email protected] (M. Wessling). 1 From Inorganic Materials Science Group.
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.02.039
very interesting. Several processes for making single layer inorganic hollow fibers have been reported in literature, including wet spinning [1–4] and dry spinning [5]. Applied materials include alumina, silicon-nitride and perovskites. Recent developments in polymer membrane synthesis have enabled the preparation of multi-layer organic hollow fibers in a single step, via a wet spinning process [6–8]. In this paper we propose a new generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning followed by a heat treatment. We will demonstrate the proof-of-principle of this concept for an ␣-alumina/␣-alumina system. 1.1. Background The wet spinning process is based on the principle of phase inversion. First a spinning mixture is prepared from a polymer, a solvent and an inorganic powder. Subsequently, the produced mixture is extruded through a spinning dye (spinneret) into a bath of non-solvent. This non-solvent, usually tap water, is also introduced through the bore of the spinning dye. Exchange of solvent and non-solvent leads to thermodynamic instability of the spinning mixture and induces liquid–liquid demixing. Further exchange leads to solidification of the polymer-rich phase, while simultaneously
266
J. de Jong et al. / Journal of Membrane Science 239 (2004) 265–269
entrapping the inorganic particles. The polymer matrix in the formed precursor fiber is removed in a heat treatment step and the inorganic particles are sintered together to give the final fiber properties. The macrostructure of the produced fiber seems to be largely determined by the structure of the precursor fiber. This structure can be controlled through the choice of composition of the spinning mixture and the conditions of phase separation. Both symmetric and asymmetric structures with different porosity and tortuosity can be prepared [9,10]. Additional advantages of the wet spinning process are the ability to work at room temperature and the high production capacity that can be achieved. The wet spinning process of double or multi-layer inorganic hollow fibers shows large similarities to the process described above, but is far more complicated. First of all, a more sophisticated spinneret design is required. In this work a triple orifice spinneret has been used, such as described by He et al. [7]. Furthermore, simultaneous spinning, and subsequent drying and sintering, introduces various complications. Especially at the interface between different layers stresses can develop, causing cracking, delamination and/or channeling. As a result the mechanical strength and separating properties of the produced membranes are compromised. Therefore, the composition of the used spinning mixtures and the parameter settings during spinning and the heat treatment are crucial. Based on prior knowledge, we chose polyethersulfone (PES), N-methyl-2-pyrrolidone (NMP) and water as polymer, solvent and non-solvent, respectively. To avoid incompatibility problems, this system was used for both layers. Polyvinylpyrrolidone (PVP) was added to suppress the formation of macrovoids during the phase inversion process and to increase the viscosity of the spinning mixture. We selected two well-defined ␣-alumina powders of different particle size, such that co-spinning of the two solutions would result in two layers, separated by a distinct interface. An important choice was the positioning of the separation layer. As this layer usually comprises smaller and therefore more reactive particles, compared to the support layer, it will show a higher shrinkage rate during co-sintering. This can lead to a stress build-up with aforementioned consequences. However, the stress might be beneficial when directed radially inwards. Therefore we applied the separation layer on the outside of the fiber.
Table 1 Composition of spinning mixtures Mixture
Inner (support layer) Outer (separation layer)
Composition PES (g)
NMP (g)
AKP15 (g)
AKP50 (g)
PVP (g)
20 20
80 90
100 –
– 55
5 –
size 0.29 m) were from Sumitomo Chemicals Co LTD. All chemicals were used without further pre-treatment. 2.2. Spinning solution preparation Polyethersulfone was dissolved in NMP. Alumina powder was added and the mixture was stirred for two days to ensure a good dispersion of the alumina powder. This is essential in the spinning process, as aggregates can lead to unstable spinning and blocking of the spinning dye. Finally PVP was slowly added. Because of the hygroscopic nature of NMP, contact with water was avoided as much as possible. Both mixtures were ultrasonically treated, to break-up any present particle aggregates. Afterwards they were degassed for a day in a spinning reservoir. The composition of the used spinning mixtures is specified in Table 1. 2.3. Spinning process The dimensions of the triple orifice spinneret are listed in Table 2. To co-extrude the spinning mixtures through the spinneret, a nitrogen pressure of 4 and 1 bar was applied on the inner and outer spinning mixture, respectively. Tap water was used as a bore liquid, and pumped through the bore of the spinneret at a speed of 1.5 ml/min. After passing an air gap of 35 mm, the fiber was immersed in a water bath for further solvent exchange to take place. After collection, the produced fibers were cut into pieces of 30 cm and rinsed with tap water for a day. Then they were allowed to dry in ambient environment before the heat treatment was applied. 2.4. Heat treatment The heat treatment was carried out in a seven-zone controlled tubular furnace and consisted of three steps. 1. Removal of any solvent still present in the polymer.
2. Experimental 2.1. Materials Polyethersulfone (PES) (Ultrason E6020P) was purchased from BASF. N-methyl-2-pyrrolidone (NMP) (synthesis grade) and polyvinylpyrrolidone (PVP) (K85–95, Mw 1,300,000) were supplied by Acros Organics. Powders used for the support (␣-alumina AKP15, particle size 0.7 m) and separating layer (␣-alumina AKP50, particle
Table 2 Dimensions of the triple orifice spinneret Spinneret dimensions Bore Support layer
dbore din dout
0.51 (mm) 0.80 (mm) 1.5 (mm)
Separation layer
din dout
1.7 (mm) 2.0 (mm)
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2. Removal of the polymer matrix by thermal and oxidative decomposition. 3. Sintering of the particles to reach the final fiber structure. Precursor fibers were sintered horizontally for 2 h at 1200 ◦ C in air. A temperature ramp of 1 ◦ C/min was chosen in both the heating and cooling step. No particular polymer burnout region was specified in the applied temperature program. The weight loss and heat flow of the fibers during the heat treatment was analyzed by thermogravimetric analysis (TGA) in combination with differential scanning calorimetry (DSC) (Setaram Setsys TG-DSC 15). 2.5. Characterization Produced precursor and sintered double layer hollow fibers were examined by scanning electron microscopy (JEOL TSM 220) at 15 kV. Cross-section samples of precursor fibers were made by cryogenic fracture after wetting, while cross-section samples of sintered fibers were made by direct snapping. Samples were dried overnight in a vacuum oven at 30 ◦ C and coated with a thin gold layer before usage. Dead-end gas permeation experiments were carried out to determine the performance of the sintered fibers. Fibers were sealed with Araldite® at one end and nitrogen pressure was applied on the inside of the fiber at the other end. Nitrogen flow was measured at different transmembrane pressures by a Brooks 5850E mass flow indicator. The permeance was determined from the slope of the flow versus trans-membrane pressure graph. An average pore size was estimated from the gas permeance data using the procedures described by Burggraaf and Cot [11]. The average volume porosity εv was determined by pycnometry (Micromeritics Accupyc 1330) using the following formulae: ρpycno − ρfiber εv = (1) ρpycno ρfiber =
4msample 2 − d 2 )l π(dout in sample
(2)
where ρ is the density, m and l the mass and length of the sample and din and dout the inner and outer diameter of the sample, respectively.
3. Results and discussion 3.1. Precursor fibers Fig. 1 presents SEM micrographs of wet-spun precursor fibers. The adhesion between the two layers is very good. The polymer matrices of both layers even seem to be connected. The inorganic particles are present in the cavities of the polymer matrix and not entrapped in the polymer itself.
Fig. 1. SEM micrographs of precursor fibers: (a) fiber; (b) close-up of outer layer; (c) interface; (d) close-up of interface.
It seems that the particles act as nuclei around which the polymer lean phase can grow. The produced fibers are nicely round and the top layer thickness varies around 40 m. Considering the spinneret dimensions with a slit of 150 m for the outer layer, the determined values are much lower than expected. This indicates either shrinkage of the layers during spinning or a mismatch in spinning speed between the layers, leading to stretching of the outer layer. The dimensions of the inner layer are in reasonable agreement with the spinneret dimensions. Therefore the second explanation seems most logic.
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Fig. 2. SEM micrographs of sintered fibers: (a) fiber; (b) close-up of outer layer; (c) interface; (d) close-up of interface.
Fig. 1 also illustrates the presence of macrovoids in both layers. This is considered unfavorable, as macrovoids negatively influence the mechanical strength of the fiber. Optimization of the recipe should lead to better results.
layers is asymmetric, although this is not as clear as in the precursor state. Produced fibers look very porous, and the degree of sintering is low. Comparing the images of Figs. 1 and 2, one can conclude that the macrostructure of the sintered fiber is largely determined by the macrostructure of its precursor: macrovoids in the precursor fiber are still present in the sintered fiber. This observation was also made by Luyten et al. [2] and Tan et al. [3]. On the other hand, differences in microstructure seem to be largely lost during the heat treatment. Another observation that can be made, is significant shrinkage of the fiber during the sintering step. The diameter of the fibers decreases from 1.4 to about 1.2 mm, while the wall thickness decreases from 0.22 to about 0.19 mm. The thickness of the separating layer decreases from 40 to 25 m approximately. In the axial direction shrinkage occurs in the range of 10–15%. The average volume porosity of the sintered fibers was found to be between 37 and 45%. The permeance of the fibers for nitrogen was around 800 m3 /m2 ·h·bar, which is reasonable compared to values reported by Liu et al. for single layer alumina hollow fibers [4]. The average pore size estimated from gas permeation data was around 0.5 m. Based on the AKP-50 particles of 0.29 m, an average pore size of roughly 0.10 m would be expected. The difference can be explained by the estimation method, which assumes a homogenous structure. As the membrane consists mainly of an asymmetric structure formed by 0.7 m particles and also contains macrovoids, a pore size larger than 0.3 m seems reasonable. Furthermore, the presence of defects in the top layer also contributes to a higher value. SEM micrographs indeed showed irregularities on the outer surface of the fibers. The shape of these irregularities suggested accumulation of gasses during the decomposition of the polymer matrix. A TGA-DSC measurement was performed to analyze this process and identify the temperature region in which it occurs. The results from this measurement are presented in Fig. 3. In the region of 450–550 ◦ C a weight loss can be detected. In the same region a peak in the heat flow is visible, indicating an exothermic reaction. It can therefore be concluded that this is the region of polymer burnout. Further optimisation of the temperature program is necessary to control the burnout process and avoid the formation of defects. A lower heating rate in the burnout region is expected to give better results.
4. Conclusions 3.2. Sintered fibers The fibers were still intact after the sintering step, but mechanical strength was poor. Breakage of fibers did not lead to separation of the two layers, indicating good adhesion. The SEM micrographs in Fig. 2 verify a good adhesion between both layers after sintering. The structure of the
In this work we have proposed a generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning combined with a heat treatment. With this new method, membranes with a high surface area per unit volume ratio can be produced, while production time and costs are dramatically reduced.
269
1.00
16
0.95
12
0.90
8
0.85
4
0.80
0 0
100
200
300
400
500
600
Heat flow [mW]
Relative weight [-]
J. de Jong et al. / Journal of Membrane Science 239 (2004) 265–269
700
Temperature [ºC]
Fig. 3. TGA-DSC thermogram of precursor fiber heated in air. Air flow rate 49 ml/min, heating rate 1 ◦ C/min.
The proof-of-principle of the concept has been successfully demonstrated with the production of double layer ␣-alumina hollow fibers. Although various problems were anticipated at the interface of the layers, the adhesion between the two layers is surprisingly good, both in the precursor and the sintered fiber. Produced fibers show an asymmetric structure with a porosity between 37 and 45%. The macrostructure of the sintered fiber is largely determined by the macrostructure of the precursor fiber, while differences in microstructure disappear during the heat treatment step. With respect to the generic concept, we found that the following requirements are crucial for successful production of defect free double layer inorganic hollow fibers. • Good dispersion of particles in the spinning solutions. • Macrovoid-free and delamination-free precursor fibers. • Controlled burnout of the polymer matrix. The proposed method is not limited to ␣-alumina membranes; in principle many ceramic or metallic powders may be used. This means that this method can open up the way for a new generation of membranes.
References [1] K.H. Lee, Y.M. Kim, Asymmetric hollow inorganic membranes, Key Eng. Mater. 61–62 (1991) 17–22.
[2] J. Luyten, W. Adriansen, I. Genné, et al., Spinning of ceramic fibers with a special structure for membrane applications, Ceram. Proc. Sci. 83 (1999) 415–423. [3] X. Tan, S. Liu, K. Li, Preparation and characterization of inorganic hollow fiber membranes, J. Membr. Sci. 188 (2001) 87–95. [4] S. Liu, K. Li, R. Hughes, Preparation of porous aluminium oxide (Al2 O3 ) hollow fiber membranes by a combined phase-inversion and sintering method, Ceram. Int. 29 (2003) 875–881. [5] J. Smid, C.G. Avci, V. Günay, et al., Preparation and characterization of microporous ceramic hollow fiber membranes, J. Membr. Sci. 112 (1996) 85–90. [6] D.F. Li, T.S. Chung, R. Wang, Y. Liu, Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation, J. Membr. Sci. 198 (2002) 211– 223. [7] T. He, M.H.V. Mulder, H. Strathmann, M. Wessling, Preparation of composite hollow fiber membranes: co-extrusion of hydrophilic coatings onto porous hydrophobic support structures, J. Membr. Sci. 207 (2) (2002) 143–156. [8] C.C. Pereira, R. Nobrega, K.-V. Peinemann, C.P. Borges, Hollow fiber membranes obtained by simultaneous spinning of two polymer solutions: a morphological study, J. Membr. Sci. 226 (2003) 35– 50. [9] I.M. Wienk, H.A. Teunis, Th.v.d. Boomgaard, C.A. Smolders, A new spinning technique for hollow fiber ultrafiltration membranes, J. Membr. Sci. 78 (1–2) (1993) 93–100. [10] S.-G. Li, G.H. Koops, M.H.V. Mulder, T. van den Boomgaard, C.A. Smolders, Wet spinning of integrally skinned hollow fiber membranes by a modified dual-bath coagulation method using a triple orifice spinneret, J. Membr. Sci. 94 (1) (1994) 329–340. [11] A.J. Burggraaf, L. Cot, Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, 1996, p. 103.
Towards single step production of multi-layer inorganic hollow fibers J. de Jong, N.E. Benes1 , G.H. Koops, M. Wessling∗ Membrane Technology Group, Faculty of Science and Technology, University of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands Received 1 September 2003; received in revised form 5 December 2003; accepted 2 February 2004
Abstract In this work we propose a generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning combined with a heat treatment. With this new method, membranes with a high surface area per unit volume ratio can be produced, while production time and costs are dramatically reduced. The proof-of-principle of the concept will be demonstrated with the production of double layer ␣-alumina hollow fibers. Although various problems were anticipated at the interface of the layers, the adhesion between the two layers is surprisingly good, both in the precursor and the sintered fiber. Produced fibers show an asymmetric structure with a porosity between 37 and 45%. The macrostructure of the sintered fiber is largely determined by the macrostructure of the precursor fiber, while differences in microstructure disappear during the heat treatment step. The proposed method is not limited to ␣-alumina membranes; in principle many ceramic or metallic powders may be used. This means that this method can open up the way for a new generation of membranes. © 2004 Elsevier B.V. All rights reserved. Keywords: Wet spinning; Co-extrusion; Co-sintering; Ceramic double layer hollow fiber; Membranes
1. Introduction Because of their thermal and chemical stability, inorganic membranes offer an attractive alternative for separation processes where organic polymer membranes cannot be used. However, inorganic membranes show some major drawbacks, including: (a) high price; (b) long and complicated production process; and (c) low surface area per unit volume (A/V ratio). The high price is not merely due to the starting materials, but can be attributed to the complicated, time and energy consuming production process, which generally comprises several sequential steps. First a support layer is made to provide mechanical strength to the membrane. Subsequently, on top of this support one or more intermediate layers have to be coated, before the final separation layer can be applied. Each step includes an expensive heat treatment. Clearly, a reduction or combination of steps is desired to cut production time and costs and thereby membrane price. The price could be decreased even further by increasing the A/V ratio of the membranes. Membranes with a hollow fiber geometry are in this respect ∗
Corresponding author. Tel.: +31 53 489 2951; fax: +31 53 489 4611. E-mail address: [email protected] (M. Wessling). 1 From Inorganic Materials Science Group.
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.02.039
very interesting. Several processes for making single layer inorganic hollow fibers have been reported in literature, including wet spinning [1–4] and dry spinning [5]. Applied materials include alumina, silicon-nitride and perovskites. Recent developments in polymer membrane synthesis have enabled the preparation of multi-layer organic hollow fibers in a single step, via a wet spinning process [6–8]. In this paper we propose a new generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning followed by a heat treatment. We will demonstrate the proof-of-principle of this concept for an ␣-alumina/␣-alumina system. 1.1. Background The wet spinning process is based on the principle of phase inversion. First a spinning mixture is prepared from a polymer, a solvent and an inorganic powder. Subsequently, the produced mixture is extruded through a spinning dye (spinneret) into a bath of non-solvent. This non-solvent, usually tap water, is also introduced through the bore of the spinning dye. Exchange of solvent and non-solvent leads to thermodynamic instability of the spinning mixture and induces liquid–liquid demixing. Further exchange leads to solidification of the polymer-rich phase, while simultaneously
266
J. de Jong et al. / Journal of Membrane Science 239 (2004) 265–269
entrapping the inorganic particles. The polymer matrix in the formed precursor fiber is removed in a heat treatment step and the inorganic particles are sintered together to give the final fiber properties. The macrostructure of the produced fiber seems to be largely determined by the structure of the precursor fiber. This structure can be controlled through the choice of composition of the spinning mixture and the conditions of phase separation. Both symmetric and asymmetric structures with different porosity and tortuosity can be prepared [9,10]. Additional advantages of the wet spinning process are the ability to work at room temperature and the high production capacity that can be achieved. The wet spinning process of double or multi-layer inorganic hollow fibers shows large similarities to the process described above, but is far more complicated. First of all, a more sophisticated spinneret design is required. In this work a triple orifice spinneret has been used, such as described by He et al. [7]. Furthermore, simultaneous spinning, and subsequent drying and sintering, introduces various complications. Especially at the interface between different layers stresses can develop, causing cracking, delamination and/or channeling. As a result the mechanical strength and separating properties of the produced membranes are compromised. Therefore, the composition of the used spinning mixtures and the parameter settings during spinning and the heat treatment are crucial. Based on prior knowledge, we chose polyethersulfone (PES), N-methyl-2-pyrrolidone (NMP) and water as polymer, solvent and non-solvent, respectively. To avoid incompatibility problems, this system was used for both layers. Polyvinylpyrrolidone (PVP) was added to suppress the formation of macrovoids during the phase inversion process and to increase the viscosity of the spinning mixture. We selected two well-defined ␣-alumina powders of different particle size, such that co-spinning of the two solutions would result in two layers, separated by a distinct interface. An important choice was the positioning of the separation layer. As this layer usually comprises smaller and therefore more reactive particles, compared to the support layer, it will show a higher shrinkage rate during co-sintering. This can lead to a stress build-up with aforementioned consequences. However, the stress might be beneficial when directed radially inwards. Therefore we applied the separation layer on the outside of the fiber.
Table 1 Composition of spinning mixtures Mixture
Inner (support layer) Outer (separation layer)
Composition PES (g)
NMP (g)
AKP15 (g)
AKP50 (g)
PVP (g)
20 20
80 90
100 –
– 55
5 –
size 0.29 m) were from Sumitomo Chemicals Co LTD. All chemicals were used without further pre-treatment. 2.2. Spinning solution preparation Polyethersulfone was dissolved in NMP. Alumina powder was added and the mixture was stirred for two days to ensure a good dispersion of the alumina powder. This is essential in the spinning process, as aggregates can lead to unstable spinning and blocking of the spinning dye. Finally PVP was slowly added. Because of the hygroscopic nature of NMP, contact with water was avoided as much as possible. Both mixtures were ultrasonically treated, to break-up any present particle aggregates. Afterwards they were degassed for a day in a spinning reservoir. The composition of the used spinning mixtures is specified in Table 1. 2.3. Spinning process The dimensions of the triple orifice spinneret are listed in Table 2. To co-extrude the spinning mixtures through the spinneret, a nitrogen pressure of 4 and 1 bar was applied on the inner and outer spinning mixture, respectively. Tap water was used as a bore liquid, and pumped through the bore of the spinneret at a speed of 1.5 ml/min. After passing an air gap of 35 mm, the fiber was immersed in a water bath for further solvent exchange to take place. After collection, the produced fibers were cut into pieces of 30 cm and rinsed with tap water for a day. Then they were allowed to dry in ambient environment before the heat treatment was applied. 2.4. Heat treatment The heat treatment was carried out in a seven-zone controlled tubular furnace and consisted of three steps. 1. Removal of any solvent still present in the polymer.
2. Experimental 2.1. Materials Polyethersulfone (PES) (Ultrason E6020P) was purchased from BASF. N-methyl-2-pyrrolidone (NMP) (synthesis grade) and polyvinylpyrrolidone (PVP) (K85–95, Mw 1,300,000) were supplied by Acros Organics. Powders used for the support (␣-alumina AKP15, particle size 0.7 m) and separating layer (␣-alumina AKP50, particle
Table 2 Dimensions of the triple orifice spinneret Spinneret dimensions Bore Support layer
dbore din dout
0.51 (mm) 0.80 (mm) 1.5 (mm)
Separation layer
din dout
1.7 (mm) 2.0 (mm)
J. de Jong et al. / Journal of Membrane Science 239 (2004) 265–269
267
2. Removal of the polymer matrix by thermal and oxidative decomposition. 3. Sintering of the particles to reach the final fiber structure. Precursor fibers were sintered horizontally for 2 h at 1200 ◦ C in air. A temperature ramp of 1 ◦ C/min was chosen in both the heating and cooling step. No particular polymer burnout region was specified in the applied temperature program. The weight loss and heat flow of the fibers during the heat treatment was analyzed by thermogravimetric analysis (TGA) in combination with differential scanning calorimetry (DSC) (Setaram Setsys TG-DSC 15). 2.5. Characterization Produced precursor and sintered double layer hollow fibers were examined by scanning electron microscopy (JEOL TSM 220) at 15 kV. Cross-section samples of precursor fibers were made by cryogenic fracture after wetting, while cross-section samples of sintered fibers were made by direct snapping. Samples were dried overnight in a vacuum oven at 30 ◦ C and coated with a thin gold layer before usage. Dead-end gas permeation experiments were carried out to determine the performance of the sintered fibers. Fibers were sealed with Araldite® at one end and nitrogen pressure was applied on the inside of the fiber at the other end. Nitrogen flow was measured at different transmembrane pressures by a Brooks 5850E mass flow indicator. The permeance was determined from the slope of the flow versus trans-membrane pressure graph. An average pore size was estimated from the gas permeance data using the procedures described by Burggraaf and Cot [11]. The average volume porosity εv was determined by pycnometry (Micromeritics Accupyc 1330) using the following formulae: ρpycno − ρfiber εv = (1) ρpycno ρfiber =
4msample 2 − d 2 )l π(dout in sample
(2)
where ρ is the density, m and l the mass and length of the sample and din and dout the inner and outer diameter of the sample, respectively.
3. Results and discussion 3.1. Precursor fibers Fig. 1 presents SEM micrographs of wet-spun precursor fibers. The adhesion between the two layers is very good. The polymer matrices of both layers even seem to be connected. The inorganic particles are present in the cavities of the polymer matrix and not entrapped in the polymer itself.
Fig. 1. SEM micrographs of precursor fibers: (a) fiber; (b) close-up of outer layer; (c) interface; (d) close-up of interface.
It seems that the particles act as nuclei around which the polymer lean phase can grow. The produced fibers are nicely round and the top layer thickness varies around 40 m. Considering the spinneret dimensions with a slit of 150 m for the outer layer, the determined values are much lower than expected. This indicates either shrinkage of the layers during spinning or a mismatch in spinning speed between the layers, leading to stretching of the outer layer. The dimensions of the inner layer are in reasonable agreement with the spinneret dimensions. Therefore the second explanation seems most logic.
268
J. de Jong et al. / Journal of Membrane Science 239 (2004) 265–269
Fig. 2. SEM micrographs of sintered fibers: (a) fiber; (b) close-up of outer layer; (c) interface; (d) close-up of interface.
Fig. 1 also illustrates the presence of macrovoids in both layers. This is considered unfavorable, as macrovoids negatively influence the mechanical strength of the fiber. Optimization of the recipe should lead to better results.
layers is asymmetric, although this is not as clear as in the precursor state. Produced fibers look very porous, and the degree of sintering is low. Comparing the images of Figs. 1 and 2, one can conclude that the macrostructure of the sintered fiber is largely determined by the macrostructure of its precursor: macrovoids in the precursor fiber are still present in the sintered fiber. This observation was also made by Luyten et al. [2] and Tan et al. [3]. On the other hand, differences in microstructure seem to be largely lost during the heat treatment. Another observation that can be made, is significant shrinkage of the fiber during the sintering step. The diameter of the fibers decreases from 1.4 to about 1.2 mm, while the wall thickness decreases from 0.22 to about 0.19 mm. The thickness of the separating layer decreases from 40 to 25 m approximately. In the axial direction shrinkage occurs in the range of 10–15%. The average volume porosity of the sintered fibers was found to be between 37 and 45%. The permeance of the fibers for nitrogen was around 800 m3 /m2 ·h·bar, which is reasonable compared to values reported by Liu et al. for single layer alumina hollow fibers [4]. The average pore size estimated from gas permeation data was around 0.5 m. Based on the AKP-50 particles of 0.29 m, an average pore size of roughly 0.10 m would be expected. The difference can be explained by the estimation method, which assumes a homogenous structure. As the membrane consists mainly of an asymmetric structure formed by 0.7 m particles and also contains macrovoids, a pore size larger than 0.3 m seems reasonable. Furthermore, the presence of defects in the top layer also contributes to a higher value. SEM micrographs indeed showed irregularities on the outer surface of the fibers. The shape of these irregularities suggested accumulation of gasses during the decomposition of the polymer matrix. A TGA-DSC measurement was performed to analyze this process and identify the temperature region in which it occurs. The results from this measurement are presented in Fig. 3. In the region of 450–550 ◦ C a weight loss can be detected. In the same region a peak in the heat flow is visible, indicating an exothermic reaction. It can therefore be concluded that this is the region of polymer burnout. Further optimisation of the temperature program is necessary to control the burnout process and avoid the formation of defects. A lower heating rate in the burnout region is expected to give better results.
4. Conclusions 3.2. Sintered fibers The fibers were still intact after the sintering step, but mechanical strength was poor. Breakage of fibers did not lead to separation of the two layers, indicating good adhesion. The SEM micrographs in Fig. 2 verify a good adhesion between both layers after sintering. The structure of the
In this work we have proposed a generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning combined with a heat treatment. With this new method, membranes with a high surface area per unit volume ratio can be produced, while production time and costs are dramatically reduced.
269
1.00
16
0.95
12
0.90
8
0.85
4
0.80
0 0
100
200
300
400
500
600
Heat flow [mW]
Relative weight [-]
J. de Jong et al. / Journal of Membrane Science 239 (2004) 265–269
700
Temperature [ºC]
Fig. 3. TGA-DSC thermogram of precursor fiber heated in air. Air flow rate 49 ml/min, heating rate 1 ◦ C/min.
The proof-of-principle of the concept has been successfully demonstrated with the production of double layer ␣-alumina hollow fibers. Although various problems were anticipated at the interface of the layers, the adhesion between the two layers is surprisingly good, both in the precursor and the sintered fiber. Produced fibers show an asymmetric structure with a porosity between 37 and 45%. The macrostructure of the sintered fiber is largely determined by the macrostructure of the precursor fiber, while differences in microstructure disappear during the heat treatment step. With respect to the generic concept, we found that the following requirements are crucial for successful production of defect free double layer inorganic hollow fibers. • Good dispersion of particles in the spinning solutions. • Macrovoid-free and delamination-free precursor fibers. • Controlled burnout of the polymer matrix. The proposed method is not limited to ␣-alumina membranes; in principle many ceramic or metallic powders may be used. This means that this method can open up the way for a new generation of membranes.
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