The Coating Material Of Gas Separation Composite Membranes

The poly(dimethysiloxane) (PDMS) is a polymer material on excellent syntheticperformance. The bond of Si-O in the main chain is so easy to internal rotation, that themolecular chains is very soft. Thus the PDMS has high permeability which could be appliedto the field of separation membranes. But the low separation factor is a disadvantage ofPDMS, for example the separation rate of oxygen/nitrogen is only 2.0.The convenient and valid method is to synthesize poly(dimethyvinylsiloxane) (PVDMS)by induct some vinyl in side group, and then improve the separation factor by cross-linking.Though the low vinyl content of PVDMS is a commercial product, the vinyl content is toofew in markets, only 0.1～0.3mol％, which could not improve the membrane performance.A series of high vinyl content PVDMS were synthesized which vinyl content was5～25mol％. The synthetic technics were investigated carefully and the chemical structurewere characterized by FTIR and ~1H-NMR. The composite membranes for gas separation wereprepared by coating the PVDMS solution. The experimental contents were comprised by thechoice of appropriate substrate membrane, the confirmation of optimum coating liquidformula and coating conditions, the investigation of the effects of the feed pressure, themolecular weights and the vinyl content in permeability and selectivity of compositemembranes, the comparison of two kinds of imported silicone rubber and the FTIRcharacterizations.The result showed that when the catalyst was KOH, the blocking agent was MM~ⅵ, theraw materials were D4 and D4~ⅵ, the best synthetic technics were reaction temperature 140℃,reaction time 180～210min, mass fraction of MM~ⅵ0.9～1.8wt％, mass fraction of KOH0.01～0.015wt％. And at the moment the product has right viscosity and stable performance.The product structure was characterized by FTIR and ~1H-NMR, and the vinyl content was thesame as expectation.The composite membrane was formed by coating the solution which was comprised by3.0g PVDMS—5mol％, 99.66g petroleum ether, 0.20g hydrogenous silicon oil, 0.14gchloroplatinic acid catalyst twice on the PEI porous substrate membrane, then vulcanizing 4hours at 80 temperature. The performance of composite membrane was best. When thepressure was increased, the permeation rate has a rising trend and the separation rate was alittle lowering. When the viscosity of PVDMS was 0.22～0.32dl/g, the vinyl content was 5-10mol％, the performance of composite membrane was good, the separation rate of oxygen/nitrogen was in the range of 2.3～2.6. Moreover, the performance of PVDMS—5mol％wasbetter than Dowe—184 of USA, and similar to Wacker—VP7660 of Germany, The separationrate of PVDMS—10mol％could reach to 2.6 which could be used in medical oxygen-enriching membrane. At last, analysis of FT-1R showed that the cross-linking reactions ofPVDMS was existent and complete, the structure of Dowe—184 was PDMS and the Wacker—VP7660 contained phenyl.