Polyimide Membranes for Ultrafiltration
Development of Thermally and Chemically Stable Polyimide Membranes and Hollow Fibers for Ultrafiltration
Tech Area / Field
- CHE-POL/Polymer Chemistry/Chemistry
8 Project completed
Senior Project Manager
Mitina L M
Institute of Highly Pure Biopreparations, Russia, St Petersburg
- Scripps Research Institute, USA, CA, La Jolla\nPacific Northwest National Laboratory, USA, WA, Richland
Project summaryThe Project is aimed at the development of inexpensive membranes and hollow fibers for ultrafiltration which are resistant to organic solvents and allow the use of a wide range of pH (1-14) and temperature intervals up to 300 °C.
Despite the fact that a wide range of polymer membranes for ultrafiltration (UF) have been successfully applied in different areas, such membranes are of limited usefulness due to narrow temperature and pressure intervals, to 200 °C and 1MPa, respectively. Such membranes are not applied for aggressive fluids, including acids, alkalies and organic solvents. All these limitations prevent application of polymer membranes in some advanced fields of industry, such as high-temperature filtration; filtration of aggressive fluids; high-temperature membrane reactors; separation, purification and concentration of polymer solutions in chemical synthesis.
To solve these problems, inorganic metal and ceramic membranes are mainly used. But, despite the fact that inorganic membranes are extremely thermally and chemically stable, they are rather expensive (cost more than 10 times exceeds that of polymer membranes); have narrow range of pore size (minimal pore size is about 100 Å, that complies with ~ 100×103 D molecular weight cut-off ) and possess low permeability caused by rather thick selective layer.
Another type of chemically stable UF membranes are membranes from regenerated cellulose resistant to most organic solvents. The main disadvantages of these membranes are: low mechanical strength, swelling in water and alkaline solutions, and inability to work at temperatures higher than 50 °C.
As a polymer for production of membranes, aromatic polyimide (AP) is proposed. It is unique thermally and chemically stable polymer which possesses high mechanical strength and is resistant to all known organic solvents of pH range 1-14.
Aromatic polyimides are synthesized in two stages. First, the polyamic acid (PAA) (prepolymer) is produced by polycondensation. Next PAA is subjected to thermal, chemical, or catalytic imidization. As imidization results in production of insoluble polyimide, it is planned to produce polymer membranes by dry-wet casting from PAA solutions using amide solvents. This method allows production of asymmetric UF membrane composed of a thin selective layer and a large-porous substructure.
When casting asymmetric hollow fiber, the solution jet through a circular spinneret gets into a precipitation bath with precipitant of composition 1, while the precipitant of composition 2 is supplied inside the fiber. Both compositions are set in such a way that the inner fiber channel covered with selective fine-porous layer is produced, while the main bulk of fiber is large-porous.
Polyimide UF membranes are produced from PAA membranes by imidization.
At both stages structure of membranes and of hollow fibers will be analyzed by calibration method. This method, developed at the institute, allows definition of main parameters of membrane structure: mean size of pores, type of pore size distribution, thickness of selective layer, presence of defects. This method is high-efficient (one fellow worker of moderate skill can characterize up to 5 membranes during a work day).
Application of this method makes possible the answer to principal questions of membrane casting, including: the influence of concentration of PAA casting solution and composition of precipitation bath (for hollow fibers – composition of precipitants 1 and 2) on properties of membranes and hollow fibers; the influence of rate of hollow fiber casting (rate of PAA solution supply to circular spinneret) on properties of hollow fibers; the influence of imidization on structure of membranes and hollow fibers, and development of the best method for imidization.
It is supposed to study thermal stability by the change of properties of membranes and hollow fibers according to time of keeping at elevated temperature. Chemical stability will be studied by preservation of structure and properties of membranes and hollow fibers after keeping in the main types of organic solvents.
A separate stage is the development of laboratory hollow fiber modules (surface of hollow fibers is up to 0.1 m2) to analyze the hollow fibers.
The final stage is to fill patent applications for the polyimide membranes and hollow fibers.
Highly skilled personnel of the institute Laboratory of Membrane Technologies who have large experience in development and analysis of polymer membranes and hollow fibers will be involved. Laboratory Researchers developed membranes and hollow fibers based on polyamide imide, the polymer closely related in structure. Laboratory researchers were participants of ISTC Project #918 “Chemically modified filters for nano- and ultrafiltration”. They were the first in the world to produce and characterize for this project the asymmetric track-etched ultrafilters.
At Research Institute of Highly Pure Biopreparations there is available equipment to produce membranes and hollow fibers, including: equipment to form flat membranes and applied membranes; semi-automatic stand to cast hollow fibers; cells of different types to characterize membranes; chromatographic equipment to determine selective characteristics of membranes and hollow fibers; and computer processing programs.
The project is in conformity with the goals and tasks of ISTC and is aimed at development of unique thermally and chemically stable polyimide membranes and hollow fibers for ultrafiltration which will find their usage in: power-saving technologies (high-temperature membrane reactors for recasting of hydrocarbons; for one-stage production of oxygen, etc.); providing nuclear safety (purification of water from colloid impurities in cooling counters of nuclear reactors); chemical synthesis (separation, purification and concentration of polymer solutions in aggressive chemical fluids), and other.