Preparation And Papermaking Evaluation Of Copolymerized Para Aramid Precipitated Fibers

Precipitated fibers have been widely concerned because of its good uniformity and concentrated length distribution.Aramid paper made of para aramid chopped fibers and precipitated fibers,is widely used in aerospace,transportation track,electrical insulation and other high-tech fields due to its excellent properties such as high strength and high modulus,high temperature resistance.With continuous development of science and technology,the performance requirements of aramid paper in high-tech fields are getting higher.However,due to the difficulty of partial micro-melting of precipitated fibers during the hot-pressing process,the interfacial bonding between chopped fibers and precipitated fibers is poor,resulting in low internal bonding force and poor mechanical strength of para aramid paper.Therefore,improving the interfacial bonding between fibers and enhancing the mechanical strength of para aramid paper are of great significance for broadening the application of para aramid paper.To improve the micro-melting adhesion between precipitated fibers and PPTA chopped fibers and the internal interface adhesion of aramid paper,this paper provides a strategy to introduce meta structure into para aramid structure by copolymerization to prepare para aramid precipitated fibers,and further prepare para aramid paper with excellent mechanical properties by vacuum-assisted filtration and hot-pressing.Firstly,the third monomer m-phenylenediamine（MPD）was introduced into low temperature solution polycondensation of p-phenylenediamine（PPDA）and terephthaloyl chloride（TPC）for co-condensation modification to obtain copolymer modified PPTA polymers（MPPTA）with different addition amounts of meta structure.By controlling the monomer molar concentration of 0.6 mol/L,the molar ratio of amine to acyl chloride of 1.005,the initial reaction temperature of-5℃and the addition amount of MPD in p-phenylenediamine（PPD）of 50-60%,MPPTA with high molecular weight was obtained to improve the mechanical strength of subsequent aramid paper.Studies have shown that MPPTA has good thermal stability,but poor solubility in other conventional organic solvents except concentrated sulfuric acid.Therefore,when the addition amount of MPD is40-90%,the prepared MPPTA that can exist in the form of solution plays an important role in the machinability of subsequent processing.Then,the prepared MPPTA solution was diluted to a certain solid content in the NMP/H₂O coagulation bath system to precipitate a series of MPPTA precipitated fibers with different MPD content by precipitation method.The thermal properties of MPPTA were characterized by DMA,DSC and TG.It was confirmed that the introduction of MPD in the molecular chain could make MPPTA produce a glass transition temperature in the range of 250-400°C.MPPTA paper based materials formed by hot-pressing at 280°C and 13 MPa for 40 min after the MPPTA precipitated fibers with different MPD additions and PPTA chopped fibers were made.The glass transition temperature range of MPPTA precipitated fibers was observed by scanning electron microscopy.The MPPTA precipitated fibers in paper based materials have undergone a certain degree of micro-melting.The more introduction of MPD in the copolymer molecular chain,the better micro-melting effect of MPPTA paper based materials.In addition,the specific surface area of MPPTA precipitated fibers with different MPD additions and the crystallization properties,dielectric properties and mechanical properties of the paper based materials were comprehensively considered.It was found that when the MPD addition was 60%,the tensile strength of the MPPTA paper based materials was the highest,reaching 32.27 MPa.Therefore,the precipitation process of MPPTA precipitated fibers with a 60%MPD addition（MPPTA-60）was further optimized.It was found that the solid content of MPPTA-60 slurry was 3 wt%,the concentration of NMP in NMP/H₂O coagulation bath system was 30%and the shear rate was5000 rpm,the MPPTA-60 precipitated fibers with appropriate specific surface area and average length was obtained,which were 38.952 m²/g and 1.185 mm,respectively.Finally,the prepared MPPTA-60 precipitated fibers were mixed with PPTA chopped fibers to make paper.The effects of paper weight and fiber mass ratio on the mechanical properties of MPPTA-60 paper based materials were further studied.The tensile strength results showed that the mechanical properties of the paper were the best when the gram weight was 90 g/m² and the mass ratio of precipitated fiber to chopped fiber was 6:4.At the same time,the hot-pressing pressure,hot-pressing temperature and hot-pressing time in the hot-pressing process were changed to obtain the optimal mechanical properties of MPPTA-60 paper based materials.Through SEM observation,mechanical properties,dielectric properties and crystallization properties tests,it was found that when the hot-pressing conditions were 13 MPa,310°C and40 min,the micro-melting degree of MPPTA-60 precipitated fibers was the highest,which was wrapped with PPTA chopped fibers,and the interfacial bonding between the fibers was excellent.Compared with PPTA paper,the tensile strength of the obtained MPPTA-60 paper based material reached 55.53 MPa and the mechanical properties of PPTA modified papers were significantly improved.The copolymerization modified para aramid paper with high mechanical properties has a greater application prospect.