POROUS MEMBRANES USING THE DRY STRETCHING PROCESS FOR ENERGY AND
ENVIRONMENTAL APPLICATIONS
S. H. Tabatabaei, F. Sadeghi, P. J. Carreau, and A. Ajji
CREPEC, Chemical Engineering Department, Ecole Polytechnique, C.P. 6079, Succ. Centre- Ville
Montreal, QC, H3C 3A7 Canada
Microporous membranes are commonly used in
separation processes such as battery separators,
microfiltration and medical applications to control the
permeation rate of chemical components. Due to the
wide range of chemical structures, optimum physical
properties, and low cost of polymers, they are the
best candidates for the fabrication of microporous
membranes [1, 2]. In this study, we investigate the
feasibility and performance of microporous
monolayer PVDF films as well as of PP/HDPE/PP
trilayer membranes using the dry cast film process
followed by stretching. This process is more cost
effective and environnementally friendly [3, 4].
cross-section obtained for the cold and hot stretch
ratios of 55% and 75%, respectively, showed larger
pores and higher porosity for the HDPE layer
compared to the PP. As the level of the applied
extension during cold stretching increased, the water
vapor transmission rate (WVTR) of the HDPE
monolayer improved while the effect was inversed
for the PP monolayer. In addition, compared to the
monolayer membranes, the multilayer ones showed
smaller WVTR. Figure 2 illustrates separate PP and
HDPE membrane surfaces.
Three different polyvinylidene fluoride (PVDF)
resins were selected to develop porous membranes
through melt extrusion and stretching. The effect of
the polymer rheology on chain elongation in the melt
state was studied. The possibility of generating a
row-nucleated lamellar crystallization for precursor
films was investigated. The arrangement and
orientation of the crystalline phase were examined by
wide angle X-ray diffraction (WAXD) and Fourier
Transform Infrared Spectroscopy (FTIR). The
extrusion conditions and the blend compositions were
adjusted to obtain uniform precursor films with
appropriate morphology. Annealing, cold and hot
stretching were consequently employed to generate
and enlarge the pores. It was found that a proper
crystalline structure of the precursor films was
strongly dependent on molecular weight of PVDF
and process conditions. Blending of two PVDF resins
having low and high molecular weights improved the
water vapor permeability of the obtained membranes.
Figure 1 illustrates two of the obtained PVDF
membranes.
Figure 1. Illustration of PVDF membranes.
Figure 2. SEM micrographs of the surface of a) PP
and b) HDPE microporous membranes.
References
1. Venugopal G, Moore J, Howard J, Pendalwar S. J
Power Source 1998; 77:34-41.
2. Dahn JR, Fuller EW, Obrovac M, van Sacken U.
Solid State Ionics 1994; 69:265-270.
3. Sadeghi F, Ajji A, Carreau PJ. Polym Eng Sci
2007; 47:1170-1178.
4. Tabatabaei SH, Carreau PJ, Ajji A. J Membr Sci
2009; 345:148-159.
For PP/HDPE membranes, the multilayer films were
prepared using cast film extrusion followed by
stretching. The effects of draw ratio (DR), cooling air
flow rate (AFR), and annealing on the crystalline
structure and orientation of the monolayer and
components in the multilayer films were investigated
using wide angle X-ray diffraction (WAXD) and
Fourier transform infrared (FTIR). Scanning electron
microscopy images of the membrane surface and
Corresponding author : abdellah.ajji@polymtl.ca
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