Study On Preparation Of Aramid Nanofiber And Its Application In Strengthening And Anti-Counterfeiting Of Specialty Paper

Cellulose-based specialty paper,such as banknote paper and security paper,is the carrier of securities.With the development of society,this kind of specialty paper plays a vital role in market transactions,commodity economy and other fields.However,due to the impact of the nature of plant fibers,there are problems such as poor water resistance,insufficient flexibility,and poor folding endurance,which restrict the durability and service life of specialty paper.Therefore,it is urgent to develop reinforcement technology to lay the foundation for high value applications of cellulose based materials.Compared to cellulose,aramid nanofiber（ANF）has unique nanoscale structure and excellent mechanical properties,as well as advantages in thermal stability,water resistance,and photoluminescence.It is expected to provide a new choice for improving the comprehensive performance of cellulose specialty paper.However,problems such as low preparation efficiency,high cost,and poor controllability of fiber quality limit the large-scale application of ANF in the field of specialty paper.In view of above problems,this paper focuses on the low-cost preparation of aramid nanofibers and their utilization in the field of specialty paper reinforcement and anti-counterfeiting.It systematically studies the evolution of fiber structure and micromorphology during the deprotonation process of waste aramid fibers,and illustrates the efficient fiber formation mechanism and performance retention mechanism of ANF.On this basis,aramid nanofibers with high strength and high modulus,large length-diameter ratio,excellent temperature resistance and wet strength were used as the reinforcement unit to study the synergistic strengthening and toughening effect on specialty paper.Meanwhile,the strengthening mechanism of nanofibers on specialty paper was revealed.In addition,based on the ultraviolet luminescence characteristics of ANF and lanthanide metal organic frameworks（Ln-MOFs）,its application in the field of specialty paper anti-counterfeiting was explored.The research content is as follows:In order to solve the problems of low efficiency and high cost of ANF preparation,a method of rapid ANF preparation by proton donor-assisted deprotonation strategy was invented,and discarded aramid fiber resources were used to replace expensive original aramid fiber as raw material for ANF preparation.The effects of discarded aramid fiber on ANF fiber formation yield,nanofiber morphology,molecular weight retention degree and thermal stability were studied.The results show that the preparation cost of ANF can be reduced by 88%compared with the traditional method.At the same time,the molecular weight of ANF was still up to 43.0 kg/mol,maintaining the intrinsic advantage of macro aramid fiber.The ANF film has tensile strength（149.2 MPa）,toughness(10.43 MJ·m^-3),excellent thermal stability（Td:542℃）and high specific surface area(65.2 m²·g^-1),making it ideal for building advanced materials.In addition,the recycled waste aramid fiber was used for the enlarged preparation of ANF,which showed that the process has high economic feasibility and universality.Aiming at the problems of weak mechanical strength and poor folding resistance and water resistance of specialty paper,a multi-approach strategy involving direct addition,in-situ growth,and coating is proposed for introducing ANF to improve the overall properties of cellulose specialty paper.The resulting 1.5 wt%ANF reinforced paper has a tensile index of 79.1 Nm/g,an impressive folding endurance of over 3500 times,and strong water resistance（a water contact angle of 85.2°）.Furthermore,through in situ SEM observation,the mechanism of the synergistic strengthening and toughening of cellulose specialty paper by ANF is explained that the unique three-dimensional nanonetwork structure and high modulus of ANF can effectively transfer stress and dissipate energy.In response to the problems of low formation index and single fluorescent anti-counterfeiting performance of traditional fluorescent anti-counterfeiting paper,natural cotton fiber（CFs）is selected as the ideal substrate for depositing lanthanide metal-organic framework（Ln-MOFs）to prepare fluorescent anti-counterfeiting paper.Anti-counterfeit paper prepared based on Eu-MOF emits red light under 254 nm UV excitation.On this basis,the introduction of ANF enhanced and toughened the flexible cellulose substrate,and the tensile index increased by 24.9%compared with the base paper.Based on the unique fluorescence color of ANF,the composite paper has dual-wavelength emission optical switch characteristics,which can switch the color at different wavelengths（red light emission at 254 nm,green light emission at 365 nm）,which is suitable for advanced fluorescent anti-counterfeiting under naked eye vision.In this study,the high value utilization technology of waste aramid fiber was developed to realize the low-cost preparation of ANF.The strength and toughness of specialty paper can be synergistically enhanced by ANF strengthening and toughening strategy.The preparation of dual-wavelength fluorescent anti-counterfeit paper was realized by using the Ln-MOFs and ANF.This study provides certain theoretical guidance and technical basis for promoting the industrial production and diversified application of aramid nanofibers,in order to promote their application in the field of specialty paper.