A new type of polymeric gas separation membrane able to operate at elevated temperatures and withstand harsh chemical environments could help expand the use of industrial separation processes. The new membrane material could potentially allow recovery of large volumes of hydrogen gas now discarded in petrochemical processing, reducing the environmental impact of refining processes.
At the 214th national meeting of the American Chemical Society September 9 in Las Vegas, NV, researchers from the Georgia Institute of Technology reported on improved thermal and chemical resistance in gas separation membranes produced by blending polyimide materials containing crosslinkable diacetylene groups.
Crosslinking Takes Place After Membranes are Formed
A solid-state crosslinking reaction initiated after formation of the membranes accounts for the improved properties. Because the reaction is believed to occur in the more ordered regions of the blend, it does not significantly increase sample density. Thus, improvements in mechanical properties and chemical and thermal resistance are realized without reducing the material's gas transport and separation abilities.
"We have jumped a very large hurdle in having a material that is chemically and thermally resistant while retaining very attractive gas transport properties," said Dr. Mary E. Rezac, assistant professor in Georgia Tech's School of Chemical Engineering. "There could be a very large commercial market, but there are a number of technical hurdles still ahead of us."
The research is sponsored by the U.S. Environmental Protection Agency, the National Science Foundation, and Georgia Tech.
Conventional Membranes Lose Strength at High Temperatures
Conventional polymeric gas separation membranes lose mechanical strength at temperatures above 100 degrees Celsius and can be damaged by reactive components in gas streams, Rezac noted. But tests show that the new polymers are stable at temperatures of more than 400 degrees Celsius, and are not significantly affected by the chemical contaminants.
Rezac and collaborators Dr. Haskell W. Beckham, Birgit Bayer, E. Todd Sorensen, and Njeri Karangu produced the new membranes by blending a non-reactive polyimide with a new diacetylene-functionalized polyimide created in Beckham's laboratory. The blend was then dissolved in methylene chloride and formed into a film using conventional techniques. Finally, it was heated to initiate a crosslinking reaction in the diacetylene-containing portion of the polyimide blend.
The researchers used polyimides based on hexafluoroisopropylidene diphthalic anhydride (6FDA) as the non-reactive component in the blend. The reactive portion contains both a hexafluoroisopropylidene bisphthalimide moiety and an aliphatic diacetylene moiety. Varying the proportions of the two materials produced blends with different degrees of thermal and chemical resistance.
Crosslinking reactions normally cause shrinkage of the material as the carbon chains link together. That has foiled previous attempts to produce stronger membranes because increasing the density of the membrane film decreases its gas permeability, noted Beckham, assistant professor in Georgia Tech's School of Textile and Fiber Engineering.
"We believe these new materials retain their gas transport properties because the crosslinkable groups can undergo a solid-state reaction without significantly increasing the density of the material," he noted. "That is a special feature of this material."
Researchers Exploring Other Polyimide Materials
Because of its cost, however, the material developed and tested by the Georgia Tech researchers is unlikely to be used commercially. But Beckham and his graduate students have produced a series of other diacetylene-containing polyimides to determine whether these special properties may be found in similar -- and less costly -- polyimide materials.
"Incorporating diacetylene groups into linear chain backbones to crosslink polymers is not itself novel," he added. "But nobody has done a systematic study of using these groups to crosslink polyimides. We want to see if the characteristics observed for the crosslinkable 6FDA-based polyimide blends are general to polyimides, or specific to this system."
Rezac believes the new membranes could make the recovery of hydrogen from petrochemical processes economically feasible. The temperature of refinery gas streams -- containing hydrogen, propane, methane and other hydrocarbons -- exceeds the thermal operating range of current membranes, and cooling those streams can cost more than the recovered materials would be worth.
Recovered Hydrogen Could be Worth Several Hundred Million Dollars
If the new system can operate at high temperatures unaffected by gas stream contaminants, the industry could recover hydrogen the U.S. Department of Energy has estimated would be worth several hundred million dollars per year.
"If we can achieve these types of separations, we can reduce operating energy, waste production, and the pollutants going into the atmosphere," Rezac noted. "The dollar value of the hydrogen is significant, but there are external issues in terms of recycling and pollution control that are just as important."
Beckham believes there may be other applications for such materials that can undergo crosslinking without shrinkage. Dental fillings and high-strength composites are two applications that would benefit from such properties.
In addition to evaluating other polyimide materials, Rezac and Beckham hope to confirm and understand how the crosslinking process can produce stronger materials without increasing density. They theorize that the linking takes place in regions of the polyimide that do not contribute to the gas permeability.
"By selectively crosslinking the more ordered regions of the polyimide that don't contribute to the gas transport anyway, the diacetylene groups allow us to tie up the structure without reducing the gas transport properties," Beckham added.