Polyacrylonitrile hollow fibre membranes for gas separation
Polyacrylonitrile (PAN) hollow fibres have been spun by a dry-jet wet spinning technique, using a commercial PAN polymer (Courtelle) redissolved in dimethylformamide (DMF). After failure to produce satisfactory porous hollow fibres from PAN/DMF solutions, a series of studies on the porous substructure of PAN cast films prepared with a variety of additives in the casting solution and at varying temperatures of the coagulation bath were carried out. A porous and flexible PAN cast film was produced when it was precipitated in water at 55 °C with CuSO4 present in the casting solution. Hollow fibres produced from a spinning solution composed of 25wt% PAN, 70wt% DMF and 5wt% CuSO4 were more porous and flexible than those produced from 25wt% PAN and 75wt% DMF spinning solution, and appeared to be more suitable for gas separation studies. The permeability of the PAN hollow fibre membranes to single gases was studied. The experimental results showed that the calculated pore radius on the surface of the fibre was in the range of 4- 32 nm. After coating with silicone rubber, the membranes showed very poor gas permeability and selectivity. Since PAN has a low intrinsic gas permeability, the low permeability observed is ascribed to a thick skin layer. The low selectivity of the membranes is related to their high surface porosity (> 10-4), or to the large pores present which are imperfectly blocked. With such fibres, little or no gas will pass through the membranes by solution-diffusion in the PAN. In order to reduce the surface porosity on the skin layer of the hollow fibres, a dualbath coagulation spinning system was used. The gas permeability of H2 in these membranes is lower than that obtained by the single bath coagulation system, while the gas permeability of the other gases, such as CO2 and CH4, were too low to measure. These results indicate that a high selectivity can be obtained by the dual bath coagulation spinning system although the selectivity is accompanied by too low a permeability, which is itself caused by too thick a skin layer. Surface modifications of PAN hollow fibre were carried out in order to modify the surface porosity of the fibres. After the treatments, the hollow fibre membranes did not give significant improvement in gas permeability and selectivity. But, when PAN hollow fibres were treated with cuprammonium hydroxide solution at room temperature, the fibres became coloured and no longer soluble in the usual solvents. The insolubility of the fibres is presumed to be due to a newly-formed crosslinked structure. The crosslinking of the fibres is reversed when the fibres are treated with EDTA solution. It has been observed that the presence of the copper in the fibres increases the tensile strength and decreases the elongation of the hollow fibres. The interaction of the PAN fibre with the cuprammonium hydroxide gave no improvement in gas separation performance but might be the basis for general acrylic fibre modification.