Synthesis Of High Concentration Spinning Dope And Preparation And Properties Of Polyimide Fiber

Due to its unique rigid aromatic ring structure,polyimide fiber has excellent mechanical properties,heat resistance,radiation resistance,and chemical corrosion resistance.It is widely used in aerospace,defense and military industry,environmental protection,and other fields.However,technology and cost are two important factors that limit the widespread application of polyimide fibers.In recent years,with the continuous breakthrough of polyimide fiber preparation technology,the problem of high cost in the process of industrialization has become particularly prominent.Therefore,on the basis of ensuring its performance,reducing the manufacturing cost is an important direction for the research and development of polyimide fibers.Improving the production efficiency of fibers is an important means to reduce costs.In order to ensure the spinnability of the spinning solution and the sufficient molecular weight of the polymer,the solid content of the current polyimide fiber solution spinning is generally less than 20%.If the concentration of the spinning solution can be greatly increased on the premise of ensuring the spinnability,it will undoubtedly be of great significance to improve the production efficiency.To this end,the subject,on the premise of taking into account the spinnability and fiber properties,carried out a low-cost improvement of the polyimide fiber preparation process.By replacing part of PMDA with 2,5-dimethylterephthalic acid（DPA）in the traditional polymer structure of pyromellitic dianhydride（PMDA）and 4,4’-diaminodiphenyl ether（ODA）,reduce the molecular weight of the polymer in the spinning dope,adjust the viscosity of the system,and prepare a spinning dope with a solid content of 30%.By adjusting the spinning process,polyimide fibers with excellent properties are successfully obtained,and the fiber production efficiency is increased by 50%,the production costs is reduced effectively.The main research contents are as follows:1.Mix DPA with PMDA-ODA solution to obtain high solid content polyamic acid（PAA）solution by copolymerization.The effects of different DPA additions on the processability of PAA solution and the properties of the corresponding prepared polyimide films were studied,and it was found that the addition of DPA could effectively reduce the problems of excessively high viscosity and difficult processing caused by increasing the solid content.The properties of polyimide films obtained by adding different amounts of DPA for polymerization under the condition of fixed dianhydride/diamine monomer molar ratio were studied.It was found that when the added DPA made the dianhydride（PMDA+DPA）and the diamine ODA in equimolar amounts,the polyimide films prepared had better comprehensive properties.When the solid content of the PAA solution was 30%,the viscosity of the PAA solution prepared by replacing the dianhydride with different DPA ratios remained between 102 Pa·s and 636 Pa·s,and the viscosity of the polymerization system was effectively controlled.It was proved by FTIR that the carboxylate amine salt would undergo ring formation after heat treatment,and it had no obvious effect on the structure and properties of the polymer.The polyimide film prepared from this polymer solution has good mechanical and thermal properties,the breaking strength can reach 112 MPa,and the modulus is 3.5 GPa;the glass transition temperature is about 420°C;the weight loss decomposition temperature is maintained between 551℃and 558℃,and the fastest thermal decomposition rate temperature is 584℃and591℃.2.Synthesize high-solid content polyimide spinning dope with a solid content of 30%and different DPA replacement amounts,and prepare a series of as-spun fibers by dry spinning.It was found that the increase of the solid content of the spinning dope can improve the spun fibers forming,the fiber cross section changes from the original flat shape to a circular shape.The research on the process parameters such as pump supply,spinning speed and spinning temperature in the process of primary fiber forming found that:the mechanical properties of polyimide fibers obtained by relatively small pump supply are better;however,reducing the spinning speed can improve the draft-ability of the fiber,which is beneficial to the improvement of the mechanical properties of the fiber;the spun fibers prepared when the spinning temperature was 215°C had better shape and subsequent processability.3.The post-treatment process of the fiber has an important influence on the final properties of the fiber.The research on the post-treatment process of the fiber found that:the mechanical properties of the fibers first increased and then decreased with the increase of the drafting temperature,when the drafting temperature was 470℃,the mechanical properties of the obtained fibers were the best;increasing the draft ratio is beneficial to the improvement of mechanical properties;compared with the dynamic cyclization,the polyimide fiber obtained by static cyclization has better draft-ability,and the maximum drafting ratio can reach 2.7 times,fiber strength reaches 0.95 GPa and modulus is 10.5 GPa;appropriate drafting ratio during the thermal cyclization of the as-spun fibers is beneficial to improve the final mechanical properties of the fibers.The polyimide fibers obtained by thermal cyclization and thermal drawing have good mechanical properties,thermal properties and service behavior.When the replacement amount of DPA is 1%,the comprehensive performance of polyimide fiber is better,the fiber strength reaches 0.9 GPa,and the modulus is 10.2 GPa;the glass transition temperature is 393℃,the 5%thermogravimetric loss temperature in N₂ atmosphere reaches 562℃,and the maximum thermogravimetric rate temperature reaches 600℃.The research on the service behavior of fibers such as acid resistance and ultraviolet radiation resistance found that,the strength retention rate of 30%H₂SO₄ treatment for 12 h is 85.4%,the strength retention rate of 0.5%Na OH treatment for 0.5 h is 96.4%,and the strength retention rate after UV irradiation for 168 h is 81.3%,the strength retention ratios were98.5%and 92.5%after thermal-oxidative aging at 260°C for 6 h and 12 h in an air atmosphere.