Research On The Microstructure And Properties Of High Performance Natural Rubber For Aircraft Tires

Natural rubber(NR)is a kind of natural polymer with excellent comprehensive properties that are incomparable to synthetic rubbers.Therefore,NR is the most ideal raw material for aircraft tires.However,there are many neck sticking problems in the aircraft tires industry of our country.In terms of raw materials,the quality of NR raw rubber fluctuates greatly due to factors such as growth conditions and production technologies.Domestic NR is difficult to meet the high-performance demand of aircraft tires,thus most of them rely on imports.However,due to the technical blockade and confidentiality of developed countries,the screening mechanism of NR for aircraft tires is still unknown.Secondly,the research on the excellent comprehensive performance of NR is not deep and comprehensive enough,so there is no evidence to follow when producing NR raw materials or seeking alternative materials through structural imitation.Previous studies have shown that the excellent comprehensive properties of NR are due to the unique natural network structure formed by the interaction between its non rubber components and rubber molecular chains.However,most of the related studies focus on raw rubber,the effect of natural network on the network structure and properties of vulcanized NR,and the related mechanism are still unclear.From the perspective of terminal products,the performance of domestic aircraft tire is still difficult to compete with that of imported tire.The aircraft tire market is monopolized by foreign aircraft companies,thus the development of high performance NR for aviation tire is also an urgent problem.In rubber industry,carbon black and other fillers are usually used to reinforce NR,but the wear resistance of domestic aircraft tire is still obviously insufficient,which limits its service life.In order to further improve the wear resistance of NR,it is necessary to scientifically correlate its wear characteristics with its structure and properties,and clarify the key factors that determine the wear resistance of NR.In addition,the traditional reinforcement methods mainly focus on the strength and wear resistance of NR,which makes little contribution to its toughness and defect resistance.From the perspective of safety and durability,these properties are also very important,but the strength and toughness of polymer materials often conflict with each other.And traditionally,aircraft tires are irreversibly covalent crosslinked by sulfur,which makes it very difficult to recycle the waste tires.In recent years,utilizing bio-inspired strategies of sacrificial bonds and structural bionics,some breakthroughs have been made in strengthening and toughening synthetic rubbers.The discovery of vitrimer also provides an ideal solution for waste rubber recycling.However,these methods generally need to be realized through unique molecular structure design,or strongly rely on complex preparation technology.It is still far away from the actual preparation of high-performance NR and aircraft tires,but it has important development prospects.In order to solve the above problems,this paper starts from the relationship between the microstructure and macro performance of NR,and systematically studies the screening of NR and its high-performance design,the main research contents and achievements are as follows:1.In view of the fact that there is no domestic special evaluation system for NR used in aircraft tire,we tested the molecular weight and distribution,protein and phospholipids contents,gel content,green strength of NR raw rubber,as well as the dynamic and static mechanical properties of vulcanized NR such as strength,strain induced crystallization,tear resistance,fracture toughness,wear resistance,aging resistance,crack growth resistance and fatigue resistance.Through scientifically relating their composition,structure and properties,we found that the rubber with high fracture toughness has better comprehensive properties such as strength,tear resistance,wear resistance and fatigue resistance.Therefore,fracture toughness can be used as a sensitivity parameter to roughly evaluate the comprehensive properties of natural rubber.This work scientifically and systematically obtained the main properties of the important rubber varieties currently widely used in China,and found the key performance parameters affecting the comprehensive properties of NR,which not only enriched the NR basic database,but also promoted the establishment of NR screening mechanism for aircraft tires.2.In order to disclose the effect of natural network on the structure and properties of vulcanized NR and its mechanism,four NR model compounds with different nonrubber components were prepared and further vulcanized.Firstly,the energy dissipation of vulcanized NR with different nonrubber components was studied.The results showed that there were sacrificial bonds composed by proteins and phospholipids in vulcanized NR.Furthermore,the aggregation state and its evolution during deformation were visualized by fluorescence labeling.It was found that the protein and phospholipid aggregates undergo strain induced disaggregation upon stretching,representing the rupture of nature network thus prove its sacrificial mechanism.Thereafter,the strength,toughness,dynamic viscoelasticity,fatigue resistance and wear resistance of vulcanized NR were studied,and the results showed that the sacrificial characteristic of natural network can significantly improve the dynamic and static mechanical properties of vulcanized NR.This part of the work reveals the sacrificial bond properties and mechanism of natural network in vulcanized NR,and its contribution to the overall properties.It also clarifies the doubt about the role of natural network in vulcanized NR,which is of great significance for the design and preparation of high-performance NR for aircraft tires.3.In view of the wear mechanism of NR and its key determining factors are still unclear,the influences of load,rate and filler on the wear performance of NR were studied.The results showed that the wear modes of the samples were different under different working conditions,and the friction coefficient and friction torque between the friction pairs were different also.Because of the larger specific surface area,smaller filling amount and other nano effects,as well as better thermal conductivity and self-lubricating property of nanofillers,the wear resistance of NR was improved significantly by nanofiller.The correlation analysis showed that the wear resistance of NR composites has strong dependence on the tear strength and fracture toughness.This part explores the friction and wear characteristics of NR,and finds out the key affecting factors,thus provides theoretical guidance for the preparation of high wear-resistant rubber materials.4.Considering it is difficult to solve the contradiction between strength and toughness by traditional methods,inspired by the natural network in NR and bionic structure design,a class of strong,tough and defect-tolerant NR/chitosan nanocomposites with unique filler structure were prepared by latex self-assembly method.The assembly process was studied and it was found that the chitosan formed an interconnected skeleton in NR.The rigid chitosan skeleton serves as sacrificial skeleton,endowing the nanocomposites with good energy dissipation ability.The morphology evolution of chitosan skeleton during deformation was visualized by fluorescence labeling method.The crack deflection and differentiation of the nanocomposite during the tensile process were further observed.The stress dissipation was analyzed by digital image analysis,and the defect tolerance mechanism of the nanocomposite was deeply understood.Compared with the traditional dispersed filler network,the skeleton filler structure is more conducive to improve the energy dissipation,strength,toughness,defect resistance and wear resistance of NR.This work uses simple preparation method to impart NR with high strength,high toughness,good wear resistance and outstanding defect resistance,which provides a new design idea of high performance NR for aircraft tires.5.In view of the problem that waste tires are hard to recycle because of sulfur covalent crosslinks,following the latex self-assembly method in the previous chapter,we fabricate a catalyst-free and mechanically robust elastomeric vitrimer by constructing a segregated structure of sodium alginate(Na Alg)in the continuous matrix of epoxidized natural rubber(ENR),and further crosslinking the composite by exchangeable hydroxyl ester bonds at the ENR-Na Alg interfaces.The esterification reaction at the interface was monitored by FTIR in situ.The swelling experiment proved that Na Alg could covalently cross-link ENR.The dynamic properties of interfacial crosslinks and the rearrangement of network topology were studied by stress relaxation and strain recovery.The microstructure and macro properties of the composites,such as energy dissipation,strength,toughness and defect resistance,were systematically characterized,and the recycling and reprocessing of the composites were successfully realized.In addition,the introduction of rigid skeleton gives NR multiple functionalities,and we systematically characterized its shape memory,humidity response abilities and the mechanisms behind.This work improves the mechanical properties of vitrimer through special structural design,and provides a new direction for the design of highperformance and recyclable rubber materials.