Effect Of Polyimide Chopped Fiber Dispersion On The Properties And Structure Of Its Composites

In recent years,as the effective reinforcing fillers for polymer composites,inorganic or organic synthetic fibers with high strength and high modulus have gradually become the focus of researchers.The polyimide(PI)fiber,which has developed rapidly since its inception,is one of the effective reinforcing fibers of polymer composites.Therefore,it has become the focus of attention in the field of science and industry,and widely used in electrical insulation,aerospace,military,and construction industries.As a kind of chemical synthetic fibers,the surface of PI fiber is flat and smooth,with more inert groups and smaller specific surface area,resulting in low interfacial activity and poor dispersibility in aqueous media.These defects affect the distribution uniformity of PI fibers in the composites,which greatly limits the further enhancement of the properties of polymer composites by PI fibers.In this thesis,the fiber dispersibility was improved by changing the water environment parameters of PI chopped fiber suspension under certain conditions.Subsequently,surface decoration of the fiber improves the interfacial affinity of the fiber and further improves the fiber dispersibility.Finally,the PI chopped fiber three-dimensional network thin paper with uniform pore size distribution was prepared via wet papermaking technology.The vacuum curing process was used to fabricate PI fiber-reinforced carbon nanotube(CNT)/phenolic resin(PR)composites with well mechanical properties and flame retardancy.Firstly,the dispersion state of PI chopped fiber suspension was investigated by changing the values of conductivity,pH,temperature and fiber concentration of the aqueous medium and optimizing them by orthogonal experiments.The dispersion properties of PI fiber suspension under different water environments were evaluated by fiber dispersion,absorbance,and Zeta potential.The optimal parameters of the water environment were determined under this experimental condition.The optimum fiber suspension concentration was optimized.The results showed that under the conditions of this experiment,when the value of pH of the water is 6.0,the water temperature is 40?,the fiber concentration is 4wt‰,and the conductivity of the aqueous solution is 10?s/cm,the dispersion properties of the PI chopped fibers are better and the pore size distribution of the fiber paper is more uniform.In order to fabricate PI chopped fibers with better interfacial properties and dispersion properties,the surface of PI chopped fibers was modified by nanocrystalline cellulose(CNC)under the catalysis of composite Lewis acid and the cross-linking agent.The fiber surface changed from the pristine smooth structure to the cross-linked structure formed by the fiber surface and the CNC,and the irregular rough coating structure formed between the CNC nanoparticles.The wettability and dispersibility of PI chopped fiber in water were improved.The dispersion of the fibers in the aqueous solution increased by 25.0%.Compared with pristine PI fiber paper,the contact angle of paper formed by PI-CNC fibers to deionized water and ethanol was reduced by 14.9o and 4.8o,respectively.After the surface treatment,the dispersion properties of the fibers in the aqueous medium were enhanced,and the paper uniformity was also improved.Although CNC surface decoration compensates for part of the properties of the fiber after alkali treatment,the excellent properties of the fiber itself have been permanently damaged due to the influence of alkali treatment on the macromolecular chain of the fiber structure.Therefore,in order to give full play to the superior properties of PI chopped fibers,a phosphate monoester(PMOE)network structure was grown on the surface of PI chopped fibers by horseradish peroxidase(HRP)catalysis in an inverse microemulsion environment and finally prepared PI-PMOEs fibers.The interfacial properties of the fibers were improved without damaging the well self-performance of the pristine PI fibers.The results showed that physical entanglements and chemical cross-links were formed between the long chains of PMOE polymers.Most of them are closely connected on the fiber surface to form a dense network,and the remaining part extends out of the fiber surface.The roughness of the fiber surface has been improved.The existence of PMOEs on the fiber surface was further proved by XPS,and the structural density of PMOEs growing on the fiber surface can be controlled by adjusting the monomer concentration.The hydrophilicity of fibers has also been improved.The contact angle of PI-PMOEs fibers to deionized water has decreased by 13.6o,while the contact angle to ethanol has decreased by 9.9o.The surface free energy increased by 34.36%,and the dispersion of the fibers in the aqueous solution increased to 75.0%.The PI-PMOEs fibers prepared by this method not only retain the excellent performance of the pristine PI fibers but also greatly improves the interface properties,wettability,and dispersion of the fibers.After the PI-PMOEs fibers with excellent interfacial properties are prepared,the PI-PMOEs fiber paper with uniform pore size distribution are fabricated with the help of wet papermaking technology.The impregnated vacuum heating curing process was used to force the PR containing silylated multi-walled carbon nanotubes(Si-MWCNTs)to come into close contact with the "hairy" surface of PI-PMOEs fiber paper to prepare PI-PMOEs fiber-reinforced Si-MWCNTs/PR composites.In the process of wet papermaking,the fiber bundles are dispersed into a single fiber to form a three-dimensional network with uniform pore size,which is conducive to the enhancement of the fiber-matrix interface affinity and greatly enhances the mechanical properties of the fiber-reinforced Si-MWCNTs/PR composite.When the composite was exposed to flame,Si-MWCNTs network and the carbides formed by resin combustion form a complete and tight protective layer,which effectively inhibits the release of pyrolysis products.The further combustion of the material was suppressed,and the fiber-reinforced Si-MWCNTs/PR composite exhibit excellent flame retardant properties.