Study On Mechanical Properties And Model Of Spandex With Different Isocyanate Index

Spandex has excellent properties such as high elongation at break,high elastic recovery,good fatigue resistance,corrosion resistance,light resistance and heat resistance.It is mainly used for the preparation of elastic fabrics,and is also widely used in medical,fire protection,communication,and other fields.When it is used as a seat belt or a safety rope or an adaptive fiber in Z-pinch,it is required to have better safety performance to avoid serious consequences caused by material failure.Therefore,this paper studies the tensile mechanical properties and models of spandex,stress relaxation and its model,thermodynamic properties,and provides guidance on the safe use of spandex from these three aspects.The main work contents and conclusions of the thesis are as follows:(1)By changing the ratio of 4,4’-diphenylmethane diisocyanate(MDI)and polytetrahydrofuran diol(PTMG),a series of spandex with different isocyanate index(R value)was obtained by dry spinning..Tensile properties test,fourier transform infrared spectroscopy test and small angle X-ray scattering test were carried out on spandex samples.It was found that the Young’s modulus,breaking strength and elongation at break increased first and then decreased with the increase of R value,reaching a maximum at the value of 1.76,which is consistent with the change of hydrogen bonding degree and microphase separation degree.As the tensile rate of spandex increased,the breaking strength increased,while the elongation at break decreased,and the Young’s modulus remained basically unchanged.(2)A two-element model consisting of nonlinear spring and a sticky pot was proposed.This model can well fit the tensile curve of spandex,and the two-element model can simplify the analysis and calculation process compared with the existing three-element and four-element models.At the same time,the parameters of the model were analyzed.It was found that the parameter viscosity coefficient η and the initial modulus c in the model were consistent with the change of the microstructure.Therefore,the model provided a new idea and way for the study on the tensile properties of spandex.(3)The effects of R value and strain on the stress relaxation properties of spandex were studied.It was found that the first stage stress relaxation rate and total stress relaxation rate decreased first and then increased with the increase of R value.When the R value was 1.76,the minimum value was obtained,but the stress relaxation rate of the second stage did not change much.With the increase of strain,the first stage stress relaxation rate,the second stage stress relaxation rate and the total stress relaxation rate all showed an increasing trend.Different models were used to fit the mathematical model and viscoelasticity.The cubic polynomial model can predict the stress relaxation of spandex.The generalized Maxwell model with n=3 can fit the stress relaxation curve of spandex from the viscoelastic aspect,which is helpful to analyze the viscoelastic properties of spandex.(4)The thermodynamic performance of spandex under small strain was studied.It was found that the entropy of the system decreased with the increase of strain when the strain occurs.The enthalpy increased with the increase of strain.The contribution ratio of internal energy to stress decreased with increasing temperature,and it was relatively small.Entropy change was the main reason for the rebound tension of spandex when it was stretched.(5)For the spandex acting on different devices,different evaluation indicators provided guidance for the safety of its use.When using spandex as a seat belt or safety rope,it was recommended to select the spandex with a high breaking strength and elongation at break with an R value of 1.76,and then used according to the specific requirements.When using the spandex as the adaptive fiber in Z-pinch,it was recommended to select the shorter first stress relaxation time and the lower second stress relaxation rate with an R value of 1.76,and then used according to specific requirements at a suitable strain.In the course of use,it was also necessary to pay attention to the change of temperature at all times to avoid the degradation of the strength of the material caused by the decomposition of spandex,which may cause serious safety accidents due to device failure.