Performance Of Imidized Soluble Polyimide And The High-temperature Mechanical Properties Of Its Composites

The development of aircraft towards high-speed,long-distance due to the rapid development of aerospace technology coming with several serious aerodynamic heating problems leading to the significant higher surface and internal temperature in the high-speed aircraft as well as the higher requirement for the lightweight structure and material heating tolerance in the long-range battling mission.Polyimide composite material has gradually became one of the promising subsitutional material for aircraft cabins and airfoil structures as compare to titanium alloys due to its extremely high heating tolerance,excellent high-low temperature mechanical performance and robust dielectric properties.However,polyimide resin suffers intrinsic problems such as low solubility,high melting point,high curing temperature,complex molding process,and poor impact resistance,which prohibits its engineering application in advanced aircraft.Therefore,several relevant researches have been announced aiming at increasing the resin solubility,lowing the melts’ viscosity,optimizing the curing process,further promoting the impact resistance and life cycle in order to unify the resin fabricating property and robust performance of composite material,broadening its application.The study herein based on the background of high temperature resistant,lightweight composite shell in the aerospace application.The systematic researches conducted herein are based on a new type of thermosetting imidized soluble polyimide resin system as the matrix,and aiming to improve the performance of polyimide composite materials.Researches included the systematic study of curing kinetics and rheological properties of imidized soluble polyimide resins,the understanding of thermal oxidative aging behavior of carbon fiber composites,the synthesis of polyimide-based tougheners and their effects on the properties of polyimide resin composites,and high temperature mechanical properties of polyimide composite materials based on machine vision.The research provides theoretical support and technical guidance for urgent engineering problems in the application process of imidized soluble polyimide composites,such as process,toughening,high-temperature performance evaluation and aging failure mechanism,and has important engineering significance.The curing process and mass transfer law of imidized soluble polyimide system are complicated,and the curing kinetics and rheological properties of the PI resin have not been reported.In the current study,the non-isothermal DSC method is used to study the curing kinetic parameters,curing property and curing cross-linking mechanism of the PI resin.The model of the isothermal engineering rheological properties of the new polyimide resin systems can be effectively established through analyzing the isothermal and non-isothermal properties of the PI resin.The curing reaction law of the PI resin is determined,the curing kinetic model which can be well fitted by the lab results is established and the PI resin curing mechanism is revealed as well.Results indicate the glassfication point(Tg)of the PI resin can reach as high as 474℃,and the 5%weight loss temperature(T5%)is around 562℃.The rheological properties of the PI resin can be well predicted based on the Engineering Viscosity model,which provides significant theoretical support for the optimization of the composite material fabrication.The T700/PI composite material can be synthesized with PI resin and T700 carbon fiber acted as substrate and reinforce material.The thermal oxidation and accelerated aging behaviors were studied through multiple thermal oxidation cycles at 450℃ and 480℃ for a thermal holding time of 60 min.Research indicates that weight loss of T700/PI composite material gradually increases with simultaneously mechanical performance decreases with the increasing of thermal oxidation cycles,but the degradation rate is lower than 15%for the mechanical performance.The dominated reason leading to the performance degradation can be seen as the surface resin molecular chain oxidation and breakage at elevated temperature,following by the exfoliation on the interface of resin and carbon fiber,as well as the increasing porosity.The isothermal accelerating aging tests for the T700/PI composite material at 280℃,300℃ and 320℃ clearly demonstrate that bending property and interlaminar shear strength cn be well maintained above 85%at an aging time within 8 days,further confirming the excellent heat resistance and aging resistance property of the composite material.One-step high temperature synthesizing method was chosen to fabricate the TPI-A RTM toughener with excellent solubility,melting property and heating resistance by selecting asymmetric 2,3,3’,4’-biphenyltetracarboxylic dianhydride(a-BPDA)and 4,4’-diaminodiphenyl ether(ODA)as monomers.T700/TPI-A/PI composite material then fabricated with the above novel TPI-A reinforcing agent,and the impact of TPI-A toughener on the rheological property,heating resistance,mechanical performance and operation performance of PI resin and T700/TPI-A/PI composite material was studied.Results denote that Tg point and T5%point of the as-sintered TPI-A toughener can reach higher to 300℃ and 550℃,and match well with PI resin regarding to the melting,heating resistance,and compatibility.Further results indicate that the TPI-A toughener can significantly increase the mechanical strength of the PI resin and T700/TPI-A/PI composite material with small impact on the heating resistance and rheological property.The corresponding tensile strength,breakage elongating ratio,bending strength and impacting strength have been increased to 25.67%,26.93%,10.89%and 45.50%,respectively.T700/TPI-A/PI composites have the highest open-cell tensile(OHT),open-cell compression(OHC)and post-impact compressive strength(CAI),with the corresponding open-cell tensile(OHT),open-cell compression(OHC)and post-impact compressive strength(CAI)increasing by 5.21%,7.87%and 7.95%respectively.In the present study,a high temperature mechanical performance evaluation method based on machine vision combined with short beam laser speckle was established in order to simultaneously obtaining multiple mechanical performance parameters of the inner surface of the resin based composite material.The recognize of high temperature mechanical strength of the T700/PI and T700/TPI-A/PI can also be determined,which significantly broaden the measuring scope of traditional mechanical performance measurement by obtaining the high temperature strain,modulus and stress,etc.This method has been further validated through standard high temperature mechanical strength testing protocol with an approximately linear correlation between positive axial uniaxial tension strain and stress was seen for the T700/PI composite material,which further providing a novel method to evaluate the heating resistance resin based composite material.Further researches demonstrate that TPI-A toughener has negligible influence on the high temperature mechanical performance of T700/TPI-A/PI through study the high temperature mechanical performance of the composite material.Specifically,the high-temperature tensile,compression and interlayer shear property can be well maintained above 90%at 350℃,while the high temperature tensile modulus and compression modulus can be also preserved above 90%at 400℃,further supporting the assumption that T700/TPI-A/PI composite material enjoy robust high temperature resistance.