Study On Fatigue Damage Mechanism And Microstructure Regulation Of Polyurethane Elastomer Under Load-Environment Coupling

Polyurethane elastomer is a type of polymer which is made of polyurethane prepolymer,chain extender and crosslinker,and has the characteristics of elasticity,wear resistance,ozone resistance,chemical resistance,and low temperature resistance and impact resistance.Polyurethane material has excellent properties and is widely used,but in the long-term service process it is easily affected by aging and fatigue,its physical properties will decline or even fail.Aging caused by environmental factors will make the polyurethane surface crack,yellow,and reduce the mechanical properties such as hardness and strength.Polyurethane elastomer in the railway industry mainly bears the power load when the vehicle passes through,and a lot of deflection and compression stress may occur,which is easy to cause fatigue phenomenon.This fatigue can deform the molecular chain of the polyurethane elastomer,which causes the polyurethane elastomer not to restore its original shape and gradually lose its elasticity until fracture.Based on the above background,this paper studied the improvement methods of polyurethane elastomers aging and fatigue through theoretical calculation,structural design,microphase regulation,and loadenvironment coupling aging.The main contents are shown as follows:Through the quantum chemical calculation method,five graft types with similar structures but different branched chain lengths were calculated using Gaussian software.The structure was optimized separately,and then the charge calculation,frequency analysis and bond dissociation energy of hydroxyl（O-H）were calculated separately for the five molecules.The calculated charge of the hydrogen atom on the hydroxyl group in the 3-（3,5-Di-tert-butyl-4-hydroxyphenyl）propionic acid（AO-f）molecule is 0.256218 e,which is the largest among the five molecules,indicating that the atom is the most attractive to the negatively charged peroxy radical among the five molecules;the dissociation energy of the hydroxyl O-H bond is 277.1769 k J · mol^-1,which is the smallest among the five molecules,indicating that the hydrogen proton is easier to bind to the peroxy radical to eliminate the free radicals.The chemical calculation of polyurethane and the synthetic processing process of polyurethane elastomer are described in detail.Four polyurethane elastomers with different isocyanate index（R）were synthesized,and the influence of microphase regulation on polyurethane performance was explored through different proportions of soft and hard segments.The results show that the initial hardness is the highest and stable when the isocyanate index is 1.0,and the tensile strength,fracture elongation and compression permanent deformation are the best.The microscopic morphology and chemical characterization indicate that at R=1.0,the reaction is most complete and the bubble hole morphology is intact with good thermal properties.PUE/AO-f with graft blocked phenol antioxidant was synthesized and the thermal weight loss curve was compared with PUE in nitrogen and air atmosphere.The activation energy of PUE and PUE/AO-f during different degradation rates was calculated by the FW-O method.At less than 20% degradation,PUE/AO-f is about 20% more active than PUE,indicating a better thermal stability of PUE/AO-f.The mechanical properties and microstructural changes of PUR and PUR/AO-f were analyzed by the thermal oxygen aging experiment and the fatigue thermal oxygen coupling aging test.After 200 days of thermal oxygen aging,PUR/AO-f tensile strength and fracture elongation are maintained relatively high,and the static stiffness remains stable.The base index is analyzed by using the Fourier change infrared test to judge the deepening of the aging degree.The glass transition temperature of the soft segments was analyzed using a differential scanning calorimeter and characterizes the evolution of the microphase partition of the soft and hard segments.The PUR/AO-f static stiffness remains well after thermosoxygen-fatigue coupling,and the dynamic mechanical performance test loss angle cut value for the PUR samples increases by 26.4%,while the PUR/AO-f samples increased by 14.8%.The surface aging phenomenon of the hot-oxygen-aged sample is separately observed by scanning electron microscopy.