Permeability Matrimid

The first, and currently only, successful solvent-permeable hyperfiltration membrane is the Starmem series of solvent-resistant membranes developed by W.R. Grace [40]. These are asymmetric polyimide phase-inversion membranes prepared from Matrimid (Ciba-Geigy) and related materials. The Matrimid polyimide structure is extremely rigid with a Tg of 305 °C and the polymer remains glassy and unswollen even in aggressive solvents. These membranes found their first large-scale commercial use in Mobil Oil s processes to separate lube oil from methyl ethyl ketone-toluene solvent mixtures [41-43], Scarpello et al. [44] have also achieved rejections of >99 % when using these membranes to separate dissolved phase transfer catalysts (MW 600) from tetrahydrofuran and ethyl acetate solutions. 

Glassy polymeric materials are often plasticized when used in gas membranes due to sorption. This can be overcome by annealing or crosshnking, however, this method does not influence the selectivity of the membrane, instead the permeability is decreased. Another method to stabilize the plasticization is to use polymer blends, as demonstrated with Matrimid 5218 and a copoly(imide) P84. The material is stabilized against carbon dioxide plasticization and the selectivity for a mixture of carbon dioxide and methane is improved. Hollow fiber membranes composed of blends of Pis with enhanced resistance towards hydrocarbons have been developed. ... 

In order to compare the performance for polymeric and carbon membranes, Figure 15.13 shows a CO2/CH4 trade-offline for P84 and Matrimid precursors and their carbon membranes as reported by Tin et al It is clear that carbon membranes possess excellent permeation properties, where both of the permeability and ideal selectivity access the Robeson upper-bound curve. Moreover, some researchers have also investigated the influence of temperature on the gas permeability.They concluded that the gas permeability values increased with the increase of temperature due to the activated process for the CMS membranes. They also found that the apparent activation energies for CO2 calculated from the Arrhenius equation Pe = Peo Qxp(-EJRT)) was much smaller than the other gas species of O2, N2 and CH4, thereby indicating that CO2 has much higher permeability. 

FIGURE 4.15 The comparison of gas permeability for pure Matrimid and nanocomposite membranes with different loadings of MgO nanoparticles (a) permeability of helium, hydrogen, and carbon dioxide (b) permeability of oxygen, nitrogen, and methane). (Adapted from Hosseini, S.S., et al., /. Membrane Sci., 302, 207-217, 2007.)... 

The influence of H2S on the CO2 permeability in the glassy polymeric membranes polysulfone and Matrimid 5218 (a polyimide) are shown in Figures 11.6 and 11.7 respectively, where the membrane was exposed to 90% N2-10% CO2 gas mixture with H2S at 500ppm. 

In EP07708077A3 (Dabou et al. 1996), gas separation polymer membranes were prepared from mixtures of a polysulfone, Udel P-1700 and an aromatic polyimide, Matrimid 5218. The two polymers were proven to be completely miscible as confirmed by optical microscopy, glass transition temperature values and spectroscopy analysis of the prepared mixtures. This complete miscibility allowed for the preparation of both symmetric and asymmetric blend membranes in any proportion from 1 to 99 wt% of polysulfone and polyimide. The blend membranes showed significant permeability improvements, compared to the pure polyimides, with a minor change in the selectivity. Blend membranes were also considerably more resistant to plasticization compared with pure polyimides. This work showed the use of polysulfone-polyimide polymer blends for the preparation of gas separation membranes for applications in the separation of industrial gases. 

A commercially available polyimide, Matrimid 5218, exhibits a combination of selectivity and permeability for industrially significant gas pairs superior to that of most other readily available polymers. Its permeation properties, combined with its processability (i.e., solubility in common solvents) makes it an ideal candidate for gas separation applications. 

Another possible way to improve the performance of a membrane relies on increasing flux for a given permeability and selectivity by simply reducing the thickness of the active layer. An example on how AFM can help in this research program is worthy of comment here. This is the procedure to make Matrimid asymmetric membranes. 

He permeation is also very sensitive to local concentration fluctuations, and thus can be used as a probe for the phase state in polymer blends [84]. In the above-mentioned system, the PSF-rich blend exhibited partial miscibility below the Tg whereas, after annealing, the PSF- and Pl-rich domains phase separated this resulted in a reduction of the permeability coefficient and showed that PI controls the absolute permeability values. It was concluded that transport in a phase-separated Matrimid/PSF is dominated by the polyimide over a wide concentration range. Assuming that the plasticization behavior may also be dominated by the polyimide, it must be concluded that only the homogeneous blend such as Matri-mid/P84 would be less susceptible to plasticization. 

The high plasticization tendency of Matrimid can be stabilized by blending vrith copolyimide P84, which is hardly affected by the sorbed molecules [85]. The CO2 concentration in the P84 film was lower than in the Matrimid/P84 and Matrimid film at corresponding pressures. It was unclear why the sorption isotherms of the Matrimid film and the blend coincided. The permeability coefficients of the blend were found to lie between the values of the homopolymers. On the basis of the... 

The CMS material used (called CMS 800-2) was formed from the vacuum pyrolysis of Matrimid 5218. The glassy polymer matrices used were Ultem 1000 and Matrimid 5218. Performances of the synthesized MMMs were evaluated for the permeation tests of pure gases (O2, N, CO2 and CH ) and the mixed gas (10% CO 90% CH ). Permeability measurement was at 50 psia upstream pressure and 35 °C. 

Fig. 8.62 Comparison of experimental data for permeability and ideal separation fector for COJ CH gas pair with the predicted values by the Maxwell and the Bruggeman model (Matrimid 5218/CMS MMMs). (From [23])...

Gas separation membranes combining the desirable gas transport properties of molecular sieving media and the attractive mechanical and low cost properties of polymers are considered. A fundamental analysis of predicted mixed matrix membrane performance based on intrinsic molecular sieve and polymer matrix gas transport properties is discussed. This assists in proper materials selection for the given gas separation. In addition, to explore the practical applications of this concept, this paper describes the experimental incorporation of 4A zeolites and carbon molecular sieves in a Matrimid matrix with subsequent characterization of the gas transport properties. There is a discrepancy between the predicted and the observed permeabilities of O2/N2 in the mixed matrix membranes. This discrepancy is analyzed. Some conclusions are drawn and directions for further investigations are given. 

Polymer matrix selection determines minimum membrane performance while molecular sieve addition can only improve membrane selectivity in the absence of defects. Intrinsically, the matrix polymer selected must provide industrially acceptable performance. For example, a mixed matrix membrane using silicone rubber could exhibit properties similar to intrinsic silicone rubber properties, O2 permeability of 933 Baiters and O2/N2 permselectivity of 2.1 (8). The resulting mixed matrix membrane properties would lie substantially below the upper boimd trade-off curve for gas permeability and selectivity. In contrast, a polymer exhibiting economically acceptable permeability and selectivity is a likely candidate for a successful polymer matrix. A glassy polymer such as Matrimid polyimide (PI) is an example of such a material because it exhibits acceptable properties and current technology exists for formation of asymmetric hollow fibers for gas separation (10). 

To explore the difficulties in practical implementation of the above concepts, mixed matrix membranes, with 20% molecular sieves (by volume), were prepared by solution deposition on top of a porous ceramic support. The ceramic supports used were Anodise membrane filters which had 200 A pores that open into 2000 A pores and offer negligible resistance to gas flow. Initially the molecular sieve media, zeolites (4A crystals) or carbon molecular sieves, was dispersed in the solvent, dichloromethane, to remove entrapped air. After two hours, Matrimid was added to the mixture, and the solution was stirred for four hours. The solutions used varied in polymer content from 1-5 wt %. The solution was then deposited on top of the ceramic support, and the solvent was evaporated in a controlled manner. The membranes were then dried overnight at 90°C under vacuum. This was followed by a reactive intercalation post treatment technique 15) to eliminate defects. This technique involves imbibing a reactive monomer (e.g. diamine) from an inert solvent (e.g. heptane) into any micro defects. Next, a second reactive monomer (e.g. acid chloride) was introduced to reactively close defects by forming a low permeability polymer. The membranes were dried again to remove the inert solvent. Individual membrane thickness was determined by weight gain and varied from 5 to 25 Jim. 

The results of the investigation are reported in Table II. The CMS-Matrimid and Zeolite 4A-Matrimid membranes give selectivities approaching those of the native polymer at best. Also, much higher permeabilities were observed than predicted by the model for both carbons molecular sieves and zeolites. These results suggest that there was improper contact between the two phases, probably due to dewetting of... 

Chung TS, Chan SS, Wang R, Lu Z, He C. Characterization of permeability and sorption in Matrimid/C60 mixed-matrix membranes. J Membr Sci 2003 211(l) 91-9. 

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