PEBAX membranes

Polyamide-polyether block copolymers (Pebax , Elf Atochem, Inc., Philadelphia, PA) have been used successfully with polar organics such as phenol and aniline [32-34], The separation factors obtained with these organics are greater than 100, far higher than the separation factors obtained with silicone rubber. The improved selectivity reflects the greater sorption selectivity obtained with the polar organic in the relatively polar polyamide-polyether membrane. On the other hand, toluene separation factors obtained with polyamide-polyether membranes are below those measured with silicone rubber. 

Poly(amide-b-ethylene oxide) copolymers were presented in 1990 as a promising membrane material [43]. These block copolymers were developed in 1972 but in 1981 began to be used for commercial purpose under the trade name Pebax , produced by ATOCHEM [44] (now ARKEMA). Another important group of segmented poly(ester)s used for membranes are block copolymers based on PEO and PBT (poly(butylene tereph-thalate), known under commercial name of Polyactive [45]. By changing the polyamide and polyether segment, molecnlar mass and the content of each block, the mechanical, chemical, and physical properties are nicely tnned as well [46]. 

It has been observed [24] that for PEG (200g/mol) modified Pebax membrane for CO2 separation the CO2 permeability increased by a factor of about 2 (from 73 to 151 Barrer) and the separation factor CO2/H2 also increased by PEG addition (50 wt.%). This enhancement was attributed to the appearance of additional ethylene oxide (EO) units and free volume increase. Higher content of EO units results in an increase in the solubility of CO2. Later, the total free volume increase and hence the increase of the permeability was demonstrated by measurements of density and by positron annihilation lifetime spectroscopy (PALS) analysis [75]. 

The novel developed membrane materials should have potential applications on a large scale. The composite membrane preparation and its evaluation therefore must present promising results before testing the membranes in a real gas mixture. Single gas (CO2, H2, N2 and CH4) measurements of Pebax /PEG blend membrane at high pressure (up to 20 bar) and at 293 K presented higher CO2 fluxes than pristine copolymer (Figure 12.14). 

This figure shows the CO2 flux as a function of fugacity, and in all samples (Pebax and blend membranes) the CO2 flux increased when the fugacity was increased from 5 to 20 bar. This is expected because in rubbery polymers, the solubility coefficient is the determinant and it strongly depends on gas condensability, especially when strong permeant-polymer interactions exist. 

A. Car, C. Stropnik, W. Yave, K.-V. Peinemann, Pebax /polyethylene glycol blend thin film composite membranes for CO2 separation Performance with mixed gases, Sep. Purif. Tech., 62, 110-117 (2008). 

CO2 Permeation with Pebax -based Membranes for Global Warming Reduction... 

It should be noted that Car et al. [10,11] (see also Chapter 12 of this volume) worked independently on similar blend manbranes, which were made of Pebax 1657, a grade of commercial poly(amide-b-ether) block copolymer with six polyamide blocks, and free PEG. Those membranes were also shown to exhibit high selectivity and permeability performances, which were attribnted to changes in both the chemical composition (i.e. higher EO content) and the morphological stmctuie (i.e. lower material crystallinity). On the contrary, Jaipurkar [12] observed CO2 permeability and selectivity improvements for the blend of Pebax 2533 with 25% of PEG 10000, bnt not for the blends with other PEG molecular weights or composition. 

Those results motivated us, in the present work, to stndy the structure, physical characteristics and gas transport properties of membranes made of different grades of Pebax block copolymers and their blends with PEG (Table 13.1). 

In line with these observations, in the second part of the chapter, cold plasma in N2 and hydrogen was used to graft amine groups onto a Pebax membrane surface in order to enhance the overall transport through the surface-modified Pebax membrane. We expect that the amine groups created on the polymer film in a N2/H2 cold plasma gas [27] promote enhanced CO2 sorption at the upstream membrane surface, and therefore enhanced CO2 selectivity in respect of nitrogen. 

Surface Modification of Pebax Membrane by Cold Plasma... 

The CO2 extraction from flue gases would be econonucally feasible if the membranes can be produced at a low price. Pebax polymers can give an opportunity to produce low cost membranes, since they are industrially produced and are easily processable by extrusion into thin films of good quality according to the website of Arkema (www.pebax. com). Such films could be laminated onto a microporous support for a production of... 

As the CO2 molecule (polarizability value of 26.5 x 10" cm, compared with 17.6 X 10 cm for nitrogen) is highly polarizable, it can interact with the polar groups in the membrane. It seems normal that Pebax 1657 is the best membrane, due to its highest content in polar ether (EO) and amide groups. However, the situation was not quite clear for the other membranes, and a more detailed study on the fine membrane structure was required. 

Figure 13.5 Surface morphology of Pebax 1657 membrane obtained by AFM in contact (a) and in friction (b) modes. The contact mode images give the surface topology...

Figure 13.6 Optical microscopy image of Pebax 1657 membrane...

The influence of temperature on the transport properties of CO2 in Pebax 1657 and 6100 extruded films was studied in the 20-40 C range. Their negative sorption enthalpies (-25 and-5kJmoF, for 1657 and 6100, respectively) can be expected from fundamentals of sorption thermodynamics in polymers [40], Negative sorption enthalpy reduces the membrane selectivity for gas treatments at higher temperature. 

Pebax 1657-based Blend Membrane Structure and Properties... 

The permeation data (Figure 13.7) indicate first a reduction in CO2 permeability from that of a pure Pebax 1657 membrane, then a steady increase in CO2 permeability. The highest performances were obtained with a blend containing 20wt.% PEG300 the CO2 permeability reached 128 Barrer and the CO2/N2 ideal selectivity was 80. 

Figure 13.7 CO2 permeability ( ) and CO2/N2 ideal selectivity coefficient (H) of the different PEC300-Pebax 1657 blend membranes, 25 °C, 4 bars...

Our sorption and diffusion data indicated an apparently more complex behaviour of the blends. We did not observe a clear increase in CO2 solubihty coefficient in the membrane with increasing PEG content but an increase in the CO2 diffusivity. As no free volume measurements were performed in the present study, we can only suggest a similar mechanism of diffusivity enhancement by PEG additive in Pebax 1657 as that proposed by Yave et al. [43]. 

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