Effect Of Stretching On Structure And Fatigue Properties Of Polyurethane Fiber And Life Prediction

At present,in the textile industry,polyurethane fiber is elastic materials being still used in a wide range.The higher elongation at break and elastic recovery rate of the fiber products bring the fatigue durability increased significantly.In order to study the effect of cyclic stretching on the polyurethane fiber,the paper respectively set these experiments: different times under the fixed elongation rate;the different elongation rate under the fixed times and the different circumstance temperature.Then,the fibers after which three different experiments were studied from the aspects of residual elongation,micro structure,thermal properties,mechanical properties and thermodynamic properties.Finally,the following conclusions are drawn.(1)The residual elongation caused by fatigue tension is mainly contributed by the slip of hard segment of polyurethane fiber,and the effect of changing the elongation on the residual elongation is greater than that of changing the stretching times.When the ambient temperature rises,the residual elongation change exacerbates.(2)By analyzing the intermolecular forces,we can see that with the increase of the number of stretching,the hydrogen bonds of polyurethane fibers decrease,and with the increase of elongation,the hydrogen bonds of polyurethane fibers decrease,then increase.The hydrogen bond reaches a minimum at a rate of 200%.The effect of rise of the ambient temperature will aggravates destruction of hydrogen bonds.(3)Through the analysis the internal aggregation state of fiber,the cyclic stretching will make the degree of crystallinity of polyurethane fiber decrease continuously,but the elongation and the ambient temperature will change the crystallinity irregularity.(4)With respect to thermal properties,a large number of stretching times(more than 4000 times)will make the thermal decomposition stability decline,while the glass transition of soft segment also decline,but the change in elongation during stretching,Which had little effect on the glass transition.(5)With respect to mechanical properties,in the cyclic stretching process,changing the the stretching times and elongation will make the mechanical properties of polyurethane fibers decreased.It can be seen from the load-elongation curve that the effect of varying the elongation rate on the mechanical properties is greater,and the tensile strength will be further reduced by increasing the temperature during the stretching,and the elongation at break will increase first,After the decline.(6)In terms of thermodynamic properties,the effect of stretching times on the thermodynamic properties of polyurethane fibers did not show a strong regularity.The thermo-mechanical properties of the polyurethane fibers with different elongation were not the same.The results showed that the fiber internal stress increased gradually with the increase of temperature in the low-fold tensile state,the Gibbs free energy reached the maximum value when the elongation is 8%.And with the increase of cyclic tensile elongation,8% of the free energy was a downward trend.The average relative molecular mass of the soft segments in the hard segment increased first and then decreased,and the maximum value was obtained when the elongation was 200%.(7)For the fatigue fracture of polyurethane fiber during cyclic stretching,it can be illustrated by molecular chain slip,micro level of entanglement,mesoscopic level formed by silver and shear bands,and macro level extended from main crack which lead to chain-fracture.For 105 D polyurethane fiber,when the elongation is 300%,the fatigue life of polyurethane is predicted to be 3.23 × 10~6 times by N-S curve.In the service process of polyurethane fiber,it is very important for the application of polyurethane fiber in the high-precision instrument to arrange the inspection and test time and select the method of fatigue testing scientifically.