DESALINATION

ELSEVIER Desalination 145 (2002) 53-59 www.elsevier.com/locate/desal

Preparation and morphological characterization of polyurethane/polyethersulfone composite membranes

Luciana Tavares Duarte, Albert0 Claudio Habert, Cristiano Piacsek Barges*

Chemical Engineering Program, COPPE, Federal University of Rio de Janeiro, PO. Box 68502, 21945-970, Rio de Janeiro - RJ, Brazil

Tel. +5.5 (21) 2562-8351; Fax +5_5 (21) 2562-8300; emails: luduarte@peq.coppe.ufrj.bl; habert@peq.coppe.ufrj.bc cristiano @peg. coppe. ufrj. br

Received 7 February 2002; accepted 11 March 2002

Abstract

This work investigated the preparation of composite membranes by simultaneous casting of polyurethane and polyethersulfone solutions, to form the top and the support layers, respectively. These membranes are intended to be used for propylene/propane separation. Polyvinylpyrrolidone was used as an additive in the corresponding solution to obtain a support with low transport resistance. The synthesis conditions were related to the morphology of the resulting membranes. Polyurethane concentration, nature of the solvent and exposure time before immersion into the precipitation bath were primarily investigated, in addition to the influence of additives, such as lithium nitrate and formamide.

Keywords: Composite membrane; Polyurethane; Polyethersulfone; Propylene; Propane; Facilitated transport

1. Introduction

Polymer based membranes have been used in a large number of gas separation processes [l] as

the separation of propylene/propane mixtures, which is specially important since propylene is raw material in the synthesis of several chemicals and polymers [2,3]. Membrane technology has

*Corresponding author.

been seen as an alternative approach to the con- ventional processes for propylene/propane sepa- ration because of the inherent membrane process characteristics, as low cost and energy consumption, and simple operation. The major inconvenient is provided by the fact that the polymeric membranes used for the separation of olefins from paraffins have been found to show the conventional trade off, as the membranes with higher permeability

Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.

00 11-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOOlI-9164(02)00366-l

54 L.T. Duarte et al. /Desalination 145 (2002) 53-59

coefficients have poor selectivity towards the olefins.

In order to improve the selectivity towards the olefins, attempts have been made to use facilitated transport membranes [4]. The most common carriers used in facilitated membranes for olefin/paraffin separations are transition metal ions, that reversibly react with the olefins through a rc-complexation mechanism, in which the olefin n-orbital interacts with the s-and d-orbitals of the metal ion. Even though all transition metals can act as olefin com- plexing agents, silver in oxidation state (I) is the most used ion, as it forms lower stability olefin complexes in comparison to the other transition metal ions.

Facilitated transport membranes for propylene/ propane separation should be constituted by a polymer containing electron donor atoms, which are able to form coordinate bonds with the transition metal cations. Besides that, rubbery polymers are most effective to act as the matrix for these kinds of membrane, as they show low barriers to bond rotation. That may provide sufficient segmental motion of the polymer chain, allowing the olefin molecule to move from chain to chain by hopping from site to site. This mechanism is shown in Fig. 1.

Due to the poor mechanical resistance presented by rubbery polymers, better performance for

Fig. 1. Schematic representation of facilitated transport Flat composite membranes were obtained by

mechanism of the solute A through a rubbery polymer simultaneous casting of two polymer solutions at matrix, adapted from [4]. room temperature, by using a double casting knife.

propylene/propane separation can be achieved by using a composite membrane, in which the rubbery polymer just constitute the skin and a vitreous polymer is used as the support.

The phase inversion by immersion precipitation technique is used to produce asymmetric mem- branes for a large number of applications. In general, composite membranes are produced in two steps: preparation of the porous support and skin deposition. However, some authors [5,6] have been investigating the simultaneous casting or extrusion of two polymer solutions to prepare composite membranes in a single step. Besides the flexibility of this technique, there is also the possibility of using different base polymers to form the top and the support layers of the com- posite membrane. The understanding of several effects involved during membrane formation by simultaneous casting of two polymer solutions is not completely clear yet.

According to Pereiraet al. [5,6], in simultaneous casting or extrusion the adhesion of the membrane layers seems to occur when there is sufficient time for the solutions to interpenetrate through each other. This is likely favored when the region close to the interface of the solutions remains stable for a longer period. In order to reach this condition, some properties related to the compositions of the top and support solutions can be changed.

The purpose of this work was the preparation of a composite membrane for propylene/ propane separation. Polyurethane is a rubbery polymer, which has shown good performance in this separation [7], and it was chosen as the top layer material. For the support layer, polyether-sulfone was chosen due to previous experience in this laboratory [5,6].

2. Materials and methods

2.1. Flat sheet membranes

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The support solution was prepared by dissolving polyethersulfone (PES, Ultrasom 1000, from Basf) in N-methyl-2-pyrrolidone (NMP, from Vetec). Then, polyvinylpyrrolid-one (PVP K90, from Fluka) was added to the solution. The com- position was kept constant, at 20,7, and 73 wt.% of PES, PVP and NMP, respectively. Polyurethane (PU, from Cofade) was used as the polymer base in the top solution, which was prepared with different solvents, additives and PU concentrations. The influence of the precipitation bath was evalu- ated and ethanol (from Vetec) and distilled water were used. The sjlnthesis conditions are shown in Table 1.

The casting solutions were immediately im- mersed into the precipitation bath. At conditions 3 and 4, the exposure time, i.e., the time before the immersion into the precipitation bath, was changed between 30 s and 5 min. The additive concentrations, at conditions 6 and 7, were changed between 0.5 and 20 wt.%. After precipitation, the membranes were kept immersed in water at 60°C overnight, for residual solvent extraction. Then, the membranes were dried by the solvent exchange technique [8].

2.2. Hollow fibers

Composite hollow fibers were produced in a triple orifice spinneret. The spinning conditions

Table 1 Synthesis conditions for the preparation of flat sheet membranes

Condition PU, Solvent Additive* Bath wt.%

Ml 10 NMP - Water M2 10 NMP - Ethanol M3 10 THF” - Water M4 20 NMP - Water M5 20 NMP - Ethanol M6 20 NMP Formamide Water M7 20 NMP LiN03 Water

‘Formamide from Vetec, and LiNO, from Aldrich “THF - tetrahydrofuran, from Vetec

Table 2 Spinning conditions

Condition Bore liquid Gap’, cm

Fl Water 4

F2 Water 10 F3 Water 20 F4 50% NMP:Water 4 F5 50% NMP:Water 10

*Distance between the spinneret outlet and the precipitation bath.

are shown in Table 2. The top solution was prepared by using 10 wt.% of PU in THF and the support solution was the same as that used to produce flat sheet membranes. Distilled water was used as precipitation bath.

The membranes were characterized by scanning electron microscopy (SEM), in a JEOL JSM 5300 microscope.

3. Results and discussion

3. I. Flat sheet membranes

Fig. 2 shows photomicrographs of the cross- section of PES support and composite membranes produced by instantaneous immersion in water bath. As it can be seen, the support presents a thin top layer and macrovoids in the sublayer. This is due to the strong interaction between the NMP, from the casting film, and the water, from the bath, which promotes an instantaneous precipitation of the layer in contact with the bath. The top layer precipitation increases the resistance for the mass transfer for solvent and non-solvent exchange, and growth of the polymer-lean phase in the under- neath layers is preferred. The nuclei will grow further until the sublayer composition reaches the region where viscous effects are important,

It can also be observed that there is adhesion between the membrane layers. However, the top and the support solutions do not interpenetrate significantly, as a border between the layers is clearly visible. When the membranes are produced at conditions Ml, M4, M6 and M7, macrovoids

56 L.7: Duarte et al. /Desalination 145 (2002) 53-59

Fig. 2. Photomicrographs of membrane cross-sections produced by instantaneous immersion in water.

growth are favored, due to the increase of the resistance promoted by the PU layer. The additives seem to have slight effect on the membrane formation mechanism.

The morphology of the membrane obtained from the PU/THF solution, condition M3, is quite different from the others. Considering that THF- water interaction is weaker than NMP-water interaction, THF flux from the top layer to the support is favored, which changes support features.

The influence of exposure time in the morpho- logies of the membranes produced at condition M3 can be observed in Fig. 3. These membranes are different from that immersed immediately in water, Fig. 2c. Adhesion between the top layer and the support is again obtained. The top layer becomes denser as the exposure time increases.

Besides that, as there is time to THF evaporate, the amount of this solvent that flows to the support decreases and the support morphologies tend to be similar to those observed in the conditions showed early (Fig. 2).

The cross-sections of the membranes produced at condition M4, under different exposure times, are shown in Fig. 4. Considering the low volatility of NMP, longer exposure time intensifies its flux from the top layer to the support, which makes the support solution stable for a longer period, favoring spinodal decomposition.

By utilizing a non-solvent that interacts weaker with NMP, and stronger with PU, as ethanol does, the upper solution precipitates slower reducing the mass transfer resistance in the early stages of the immersion processes, which reduces the

L.7: Duarte et al. /Desalination 145 (2002) 53-59 57

1 min (a)

5 min (d)

Fig. 3. Photomicrographs of membrane cross-sections produced at condition M3, under different exposure times.

Fig. 4. Photomicrographs of membrane cross-sections produced at condition M4, under different exposure times.

macrovoids incidence in the support layer, as shown in Fig. 5. This delayed precipitation also favors the interpenetration of the solutions, Fig. 5c. In the membrane produced at condition M2, Fig. 5b, the influx of ethanol promotes a dilution

of the top layer solution changing the phase separation mechanism to nucleation and growth of polymer-rich phase. This effect was verified by a large incidence of PU spheres on the membrane surface at SEM analysis.

58 L.7: Duarte et al. /Desalination 145 (2002) 53-59

suppofi (4 condition M2 (b) :ondition M5 (c)

Fig. 5. Photomicrographs of membrane cross-sections precipitated in ethanol.

3.2. Hollow fibers

The morphologies of the support layer in hollow fibers do not seem to be influenced by the gap between the spinneret and the precipitation bath. This behavior is verified in both concentrations of NMP in the bore liquid - zero and 50 wt.%. Fig. 6 illustrates the hollow fibers produced at spinning conditions Fl and F4, without top layer solution.

to modify this tendency, even though it reduces

the length of the macrovoids due to the decrease

of the NMP flux to the bore liquid. The PU top layer in hollow fibers imposes an

additional resistance to mass transfer through the external surface and the precipitation bath, favoring the growth of macrovoids from the internal side. This is shown in Fig. 7, which presents the hollow fibers produced at conditions Fl and F4. The depth of macrovoids from internal side are smaller in the case of the hollow fibers produced with NMP in the bore liquid, for the same reasons discussed before.

In all spinning conditions investigated, macro- voids growth takes place from the external and internal sides of the hollow fibers, due to the instantaneous precipitation in both of the sides. The NMP added to the bore liquid is not sufficient

bore liq. = water (a)

Fig. 6. Photomicrographs of hollow fiber cross-sections produced at conditions Fl and F4, without top layer

bore liq. = 50% NMP:water (b) solution.

L.7: Duarte et al. /Desalination 145 (2002) 53-59 59

bore liq. = water (a)

4. Conclusions

bore liq. =

The obtained results show that it is possible to produce PLVPES composite membranes, with adhesion between the top layer and the support. However, the solutions interpenetrate weakly and a border between the layers is clearly visible. On the other hand, by utilizing ethanol as non-solvent, bath delayed precipitation is promoted and the top layer and support solutions interpenetrate more easily.

Other synthesis conditions to produce com- posite membranes should be investigated, seeking for membranes with satisfactory morphologies for propylene/propane separation. A more specific study should also be performed in order to have more comprehension about the phenomena involved in membrane formation.

Acknowledgements

The authors would like to thank CNPq/ Brazil for the doctoral scholarship of Luciana Tavares Duarte .

NMP:water (b)

References

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