Step In The Preparation Of Polyimide Fiber And Its Performance Study

Polyimide(PI) fiber is a well-known class of high performance organic fiber with highstrength and modulus, high and low temperature resistance, excellent resistance to radiation andelectrical insulation properties. So it has broad application prospects in the fields of atomicenergy industry, aerospace, national defense, highway, new construction, ocean development,sports and protective equipment, environmental protection industry and so on. However, theconventional polyimide is almost insoluble in any solvent, but also its melting point is very high,even it has no melting point, so melt-spinning and solution-spinning of polyimide is reallydifficult, which limits the industrial application of polyimide fiber. The type of commercialpolyimide fiber is few. One-step method spinning and two-step method spinning are commonlyused in polyimide solution-spinning. Two-step method solves the problem of spinning difficulty,but the fiber has poor mechanical properties because of the defects of fiber. One step methodavoids microporous problem of fiber caused by release of water molecules in the prossce ofimidization treatment, so the polyimide fiber maintains a relatively high mechanical properties.But it does harm to the human body and environment because of toxic phenolic solvents. Basedon the laboratory’s research, a polyimide with good solubility was synthesized in conventionalnon-toxic solvents, and the polyimide spun fiber was getten by dry-wet spinning of polyimidesolution, then the good performance polyimide fiber was prepared after heat treatment.Polyimide solution was synthesized by diamine (BAPM) and pyromellitic anhydride(PMDA) in N-methyl pyrrolidone (NMP) solution. The effect of imidization temperature andtime on imidization degrees of polyamic acid were studied by FTIR and TGA. The result showsthat imidization temperature of polyamic acid is about195℃, and the polyamic acid has beenbasicly completely imidized after8hours. The effect of solid content of the solution, imidizationtime, solvents, intrinsic viscosity of Polyamic acid(PAA) and other factors on the intrinsicviscosity of polyimide were investigated. The result shows that the intrinsic viscosity ofpolyimide reaches a maximum of0.997dL/g, when the solid content is16%, the solvent is NMP,the imidization time is8hours. The mechanical properties of polyimide film were tested, and theresult shows that strength and modulus gradually increased with the increasing imidization time.The imidization kinetics of the polyamide acid were studied by thermogravimetry. The resultshows that the imidization of polyamic acid occurs mainly in the120°C~250°C range. Thereaction has two stages: early rapid and late slow.The rheological behaver of spinning solution was studied by rotating rheometer. The resultshows that the spinning solution is typical of pseudoplastic non-Newtonian fluids. Apparentviscosity of solution increases with the increasing concentration of solution, decreases with increasing temperature. The loss modulus and storage modulus of spinning solution increasewith the increasing concentration of solution, decreases with increasing temperature. The internalfriction of solution increases with increasing temperature and decreases with concentration ofsolution. The polyimide spun fiber was prepared by dry-wet spinning of solution. The effect ofcoagulation bath on the morphology and mechanical properties of spun fiber was studied. Theresult shows that the spun fiber with uniformly dense structures and good mechanical propertiescan be prepared by adjusting the coagulation system (ratio of water and NMP). When the ratioof water and NMP is8:2, the temperature of coagulation bath is between5℃~15℃, the ratio ofstretching is3times, the tensile strength and initial modulus of spun fiber respectively reachs0.82cN/dtex and20.47cN/dtex.In order to obtain polyimide fibers with better mechanical properties, the spun fiber mustbe heat treated. The internal macromolecular chain of polyimide fibers can rearrange andorientate after heat treatment. The effect of heat treatment conditions on physical properties,mechanical properties, crystallinity, thermal properties and other properties of the fiber werestudied. The result shows that the color of fiber gets golden or yellow from white, and the colordeepens with the increasing temperature. The surface of fiber is more smooth, the degree ofimidization further improves, the mechanical properties greatly improves after heat treatment.The tensile strength and initial modulus respectively reaches2.78cN/dtex and59.14cN/dtex afterheat-treated in nitrogen,310℃,30min. The polyimide fiber was thermally stable at about510℃...