The Preparation Of Aramid Nanofibers Paper And Study On Its Ultraviolet Aging Resistance And Application

Aramid nanofibers（ANFs）,as a novel nano-polymer material developed in recent years,have both special nanostructure of nanofiber and excellent mechanical properties,insulating property and high-temperature resistance of aramid macrofiber per se,and thus ANFs have become one of the promising―building blocks‖in constructing high-performance composite materials.However,ANFs are such kind of organic polymer that has the same chemical structure as aramid macrofibers,possessing the potential ultraviolet（UV）aging behaviour due to the chromogenic groups in their chemical structure.Such behaviour will inevitabely influence the durability of ANFs-based materials in outdoor environment.Therefore,it is quite crucial to explore the possibility that whether ANFs with special nanostructure have better UV weatherability than aramid fibers.Aiming at solving the problems of low species diversity and unclear ultraviolet aging resistance of ANFs,in this work,four kinds of differentiated ANFs and ANFs paper were first prepared by alkali dissolution using the materials with different morphology and specific surface areas.The micromorphology,mechanical properties and thermal stabilities of ANFs and ANFs paper were analyzed.Besides,the forming and paper-forming mechanism of ANFs were preliminarily explored.Then structure and property evolution and aging mechanism of ANFs paper induced by ultraviolet light were investigated.During this process,the UV-resistance of ANFs paper and aramid fiber paper were also compared and studied.Finally,basing on the excellent UV-shielding and enhancement properties of ANFs,ANFs were introduced into nanofibrillated cellulose（NFC）matrix to explore their effects in material performances including mechanical properties,surface wettability and UV-shielding performance.The final experiment results are as follows:Firstly,four kinds of differentiated ANFs and ANFs paper were respectively prepared by alkali dissolution and wet paper-making when four types of aramid macrofiber materials（aramid yarn,chopped,pulp and fibrid）were employed.It turned out that that aramid macrofibers with larger length tended to form ANFs with larger length,and the fibers with larger specific area were easy to generate ANFs with smaller diameter size.However,the overlong ANFs were easy to accumulate and formed ANFs paper with lower break strength.Furthermore,ANFs paper prepared by ANFs with smaller diameter size had higher modulus but lower flexibility and strain at break.Such results could be attributed to the fact that the key point to prepare ANFs through alkali dissolution was the KOH/DMSO system could break the hydrogen bonding of amide bonds in ANFs along the fiber length direction.Consequently,the N-H bonds in amide groups were broken,deprotonated and then negatively charged.Gradually,aramid fibers were gradually spilt from the fiber diameter direction into nanofibers.Moreover,material fibers with larger specific area had the higher solvent permetation rate and fiber degradation rate.More interestingly,the broken N-H bonds could be rebuilt by adding proton donors such as water,and thus ANFs were reprotonated to form two-dimensional transparent paper structure through hydrogen bonding.Comprehensive analysis showed that the chopped ANFs had wider diameter distribution of 16.74±2.37 nm and bigger length size of 8¹0μm,generating the chopped ANFs paper with the best comprehensive performanes of good transparency（81.85%at 600 nm）,high strength（strength at break of 103.4±6.93 MPa）,ductility（strain at break of 5.56±0.66%）and high-temperature resistance(TG_10%of 521℃).Then the UV aging mechanism and the evolution of structure and performances of ANFs paper were explored by UV weatherability test.It showed that the break strength of ANFs paper decreased from 111.83 MP to 46.10 MPa within 96 h,and then increased to 76.55 MPa with the increasing UV aging time but less than 168 h.When refered to the micromorphology of ANFs paper,the fibrous ANFs on paper surface first degraded into ANFs nanoparticles within 96h,and then the nanoparticles became accumulated when the aging time increased but was less than 168 h.After that,the aging mechanism of ANFs paper was further explored,and it was found that the amide bonds in ANFs induced by ultraviolet light were photodegradated into carbonyl products within 96 h,but later acid anhydride formed by photochemical crosslinking of carboxylic acid in the late aging stage.Consequently,an anti-growth trend in mechanical properties,crystallinity and surface wettability of ANFs paper were observed during the aging process of ANFs paper.In addition,nine-year ultraviolet aging resistance results of aramid paper and ANFs paper showed that ANFs paper had high strength retention,which was 58.5%higher than that of aramid paper.The result was due to the special nanostructure of ANFs and the denser structure of ANFs paper.Therefore,ANFs can serve as a kind of excellent UV-shielding/absorption material to some extent.Lastly,basing on the excellent UV-shielding and enhancement performances of ANFs,a series of transparent and UV-shielding ANFs/NFC composite nanopaper was prepared by wet paper-making after homogeneous mixing.The mechanical properties,surface wettability and UV-shielding performance of ANFs/NFC composite nanopaper could be adjusted by changing the additions of ANFs.Research showed that ANFs/NFC composite nanopaper with ANFs addition of 2.0 wt.%had the best comprehensive performance.Compared with the pure NFC nanopaper,the break strength,modulus and strain at break of paper above increased by 73.41%,77.40%and 12.32%,respectively.What’s more,its surface contact angle reached to 83.9°,which was 19.86%higher than that of pure NFC nanopaper,and also its transmittance of visible light was up to 83.45%at 550 nm,and UV-shielding ratio was up to 88.64%at300 nm and 83.31%at 225 nm.After the continuous UV irradiation at 365 nm over 12 h,the composite nanopaper above still kept the good UV stability.