Abstract: Aromatic polyester fibers having high strength and high modulus of elasticity can be obtained stably for a long period of time by melt spinning by using a screw type extruder having a screw which has a compression ratio of 2.5 to 4.0 and thereby inhibiting entrapment in the resultant molten polyester of gases present in or on prowders, granules or pellets being melted, and by back pressure of the molten polyester sending gas generated by decomposition of the molten polyester to a back portion of the extruder, and then extruding and melt spinning the molten polyester at a temperature of 280.degree. C. to 420.degree. C.

Abstract: In a process for producing an aromatic polyester fiber by heat-treating a fiber obtained by melt-spinning an aromatic polyester exhibiting anisotropy in its molten state, an aromatic polyester fiber having high strength and high modulus of elasticity can be produced with suppressed fusion between single filaments during the heat treatment by (a) subjecting the fiber obtained by melt spinning to heat treatment in an organic liquid heating medium or (b) adhering to the fiber obtained by melt spinning at least one member selected from the group consisting of suspensions having a concentration of 0.05 to 20% by weight of organic polymer powders having a melting point or a softening temperature higher than the heat treatment temperature, and suspensions having a concentration of 0.1 to 20% by weight of barium sulfate powder or barium titanate powder, and then subjecting the resulting fiber to heat treatment.

Abstract: An aromatic polyester showing anisotropy in the molten state is melt spun stably for a long period of time by controlling an extrusion pressure at a nozzle portion to 3 kg/cm.sup.2 G or higher and an atmospheric temperature T.sub.N at a nozzle outlet to a degree satisfying the following formula: (T.sub.F -100).ltoreq.T.sub.N .ltoreq.T.sub.F, wherein T.sub.F is a flowing temperature of the aromatic polyester, and preferably controlling the pressure of a polymer melt at a tip outlet of an extruder to 15 kg/cm.sup.2 G or higher, whereby a highly orientated fiber of fine denier is obtained.

Abstract: Polyacrylonitrile (PAN) fiber of high strength (tensile strength.gtoreq.20 g/d) produced from a polymer composed mainly of acrylonitrile (AN) and having a weight average molecular weight not less than 400,000, and a method of producing said fiber characterized by a multistage stretching step and a drying step under particular conditions.

Abstract: The invention provides a polyacrylonitrile fiber having a tensile strength above 13 g/d and a modulus of elasticity above 2.4.times.10.sup.11 dyne/cm.sup.2, which is useful for tire cords, fiber-reinforced composite materials, precursors for producing carbon fiber, etc. The fiber is produced by integrally combining technical means which comprise using a polymer composed mainly of acrylonitrile whose weight average molecular weight is more than 400,000 and Mw/Mn ratio is less than 7.0; preparing a spinning solution from the polymer; spinning the solution into filaments; subjecting the resulting filaments to multistage stretching; and drying the filaments, all these steps being conducted under particular conditions.

Abstract: Polyacrylonitrile (PAN) fiber of high strength (tensile strength.gtoreq.20 g/d) produced from a polymer composed mainly of acrylonitrile (AN) and having a weight average molecular weight not less than 400,000, and a method of producing said fiber characterized by a multistage stretching step and a drying step under particular conditions.

Abstract: A process for producing hollow acrylonitrilic separation membranes in fibrous form which comprises shaping a membranous product in hollow fibrous form from a solution of acrylonitrile polymer dissolved in formamide-containing dimethylformamide while maintaining the temperature of said polymer solution within a specified range and subjecting said membranous product to a solvent removal treatment under a specific condition, thereby producing an acrylonitrilic separation membrane which is asymmetric, porous in structure, and excellent both in separation ability and in mechanical strength.

Abstract: A process for producing polyacrylonitrile membranes for use in reverse osmosis, which comprises dissolving an acrylonitrile polymer in formamide (FA)-containing dimethylformamide (DMF) to prepare an acrylonitrile polymer solution for membrane formation; shaping the solution into a membranous product of a desired form while maintaining the temperature of the solution within a specific range; removing the solvent from the membranous product under a specified solvent removal condition (solidifying condition); subjecting the thus-obtained membrane to a heat treatment, and further to a plasma treatment. The polyacrylonitrile membrane thus obtained is asymmetric and porous in structure, and excellent both in separation ability and in mechanical strength. This membrane is very suitable for use in reverse osmosis.