| High performance fibers (HPFs) are widely used in aviation, aerospace, military, sports, automotive, construction and other industries, because of their unique physical and chemical properties. In addition to requirements of fiber with high strength, high modulus, light weight in the common use, said fibers have good light-thermal stability and dynamic thermal mechanical behavior. Retention of shape and bearing of the HPFs are obtained in harsh environments, such as high and low temperature, high radiation, light-thermal ageing, high humidity, corrosive liquids or combinations thereof. Therefore, the HPFs are simulated and characterized in the complex environment, for obtaining dependable practical evaluation, and scientific guidance is provided for manufacture of the HPFs and formation of product thereof. In the paper, the mechanical behavior and ageing characteristics of several HPFs are characterized and analyzed under the condition of mechanical-light-thermal ageing.Ultra-high molecular weight polyethylene(UHMW-PE), Kevlar(?) and Nomex(?) are discussed by dynamic mechanical analysis(DMA). Thermo-mechanical relaxation spectrum of three fiber is analyzed, in which relaxation peak of UHMW-PE fiber appears at near-130℃,-60℃and 90℃, followed byγ,βandαrelaxation, in which a relaxation corresponds to the relaxation of crystal, and the corresponding temperature is the transition of crystallization.γcorresponds to the secondary transition of amorphous part, which belongs to movement of the end groups and side groups.βcorresponds to the glass transition of amorphous part. After thermal ageing, crystalline structure and transition temperature of the UHMW-PE are measured and analyzed by DSC and XRD, and decreasing trend of monoclinic crystalline phase is observed.Effect of thermal ageing treatment on dynamic thermal mechanical of Kevlar(?) and Nomex(?) fiber is analyzed. The most obvious difference is thatγtransition temperature is based on loss tangent tanδbetween the Kevlar(?) fibers by heat treatment of 200℃and original sample. D-value of 28.2% is found, and minimal D-value is only 1.1%, which based on theβtransition temperature of loss modulus E.The most obvious difference is that/transition temperature is based on loss modulus E" between the Nomex(?) fibers by heat treatment of 200℃and original sample. D-value of 36.7% is found, and there is no minimal D-value based on theβtransition temperature of storage modulus E". For the three transition temperature, said difference of secondaryβtransition temperature is greater than that of the other two transition temperature. The difference ofαtransition temperature is less than that ofβtransition temperature, and close to that ofγtransition temperature.Effect of mechanism of heating rate and frequency on relaxation spectrum is analyzed. With the increase of heating rate, the transition temperature towards the high temperature, and difference of up to 30℃is measured because of hysteretic properties of relaxation behavior. The frequency dependence lies on comparison with Arrhenius law and Vogle-Fulcher law. Law fitting results of Vogle-Fulcher is significantly higher than that of the traditional Arrhenius law. Relationship between the log of frequency and the reciprocal of glass transition temperature inclines to non-linear.Changes of shape and performance of Kevlar(?), Terlon(?), Nomex(?) and PBO fiber are discussed. Retention rate of mechanical properties of PBO is greater than that of Terlon(?) fiber, and retention rate of mechanical properties of Terlon(?) fiber is greater than that of Nomex(?) fiber. After light treatment, the strength retention of Terlon(?) fiber and Kevlar(?) fiber is the best. The strength retention of the light ageing is significantly higher than that of the thermal ageing, in which the difference between PBO fiber and Nomex(?) fiber is the most. After the light treatment of 200h, retention rate of breaking strength and elongation reaches 20 to 30%. While by the thermal ageing of 300℃, retention rate of both reaches above 70%, which is related to the difference between light and thermal ageing. Thermal ageing is the physical softening process of fiber, while light ageing is the process of light oxidation and light degradation and so on. Destruction of light ageing on the fiber is significantly stronger than that of the thermal ageing.By X-ray diffraction and IR testing, changes of crystal structure and chemical structure under the condition of light and thermal ageing are analyzed. With the heat treatment temperature increases, crystallinity of Kevlar(?) fiber increases, and interplanar distance retains basically unchanged, while the crystalgrain size grows larger. Crystallinity of Terlon(?) and Nomex(?) fiber decreases, in which crystalline region of Nomex(?) fiber has been greatly damaged under the condition of high-temperature. While crystallinity of the PBO fiber firstly increases and then decreases comparing with the other three fibers, and its crystallinity under the condition of high-temperature is very high up to 80%. By comparing and calculating change index of before and after ageing shows light resistance stability of Terlon(?) fiber is larger than that of Kevlar(?) fiber, and Kevlar(?) fiber is larger than that of Nomex(?), and Nomex(?) fiber is larger than that of PBO fiber.Effect of light and thermal ageing on characteristic points temperature, activation energy and the energy level of Terlon(?) fiber and Kevlar(?) fiber is analyzed. Peak temperature of Kevlar(?) fiber in the nitrogen atmosphere is larger than peak the temperature in the air atmosphere about 50～70℃. With thermal ageing temperature increases, the initial temperature of he Terlon(?) fiber in the process degradation tends to decrease, reducing nearly 40℃. By light treatment the degradation rate is lower than that of the original sample, which decreases from 11% to about 2.5% in the light treatment of 200h. With increasing light time, activation energy and reaction order gradually reduces, and the maximum D-value of the activation energy is 47kJ/mol. Reaction order decreases from 1.18 to 0.14. In the light treatment of 60h and 200h, the thermal degradation rate of Kevlar(?) fiber decreases, and In the light treatment of 100h degradation rate is even higher than that of original sample about 5%.By thermal gravimetric on Kevlar(?) fiber analysis, three methods of the Coats-Redfem, Friedman and Freeman-Carroll are compared. The results show that Coats-Redfem method is more close to the measured thermal degradation behavior of Kevlar(?) fiber, but n(n=1 or n≠1) values of reaction order is previously presumed. In order to accurately calculate△E of Kevlar(?) fiber, the activation energy△E equation is selected. The linear correlation of Friedman method is relatively lower, but it is easy and direct, and the reaction order is higher up to 1.80, to be explained. Start and end of the reaction region and expression of the dynamic parameters equation of the thermal degradation based on the slope of the TG track of Freeman-Carroll method is consistent with Friedman method. The reaction order can be analyzed and determined, and the degree of linear correlation increases, but the reaction order is smaller, up to 0.35, which need to be verified. Meanwhile, effect of the different thermal ageing temperature on the degradation of Terlon(?) fiber is analyzed, and the weight loss rate of the fiber is greater than that of original sample.The similarities and differences of the shape and characteristics for the HPFs are studied under the condition of light-thermal ageing comparing with single condition of light or thermal ageing. Mechanical retention of Kevlar(?) fiber is best. Light and heat resistance of Terlon(?) fiber is good in a short time, but under condition of the strong light and heat, its mechanical performance loss is relatively large. Damage of Nomex(?) fiber thermal is also smaller. Light and heat resistance of the PBO fiber is worst.Under the conditions of heat and light ageing the differences between calculated value and measured value of linear superposition are discussed and analyzed. The calculated value of Kevlar(?) fibers is higher than the measured value about 20%. The measured value of 61.4% of Terlon(?) fiber is also lower than its theoretical value of 79% of line superposition, and difference between the measured value of Nomex(?) fiber and the theoretical value is the most close to 40%. The measured value of PBO fiber is lower than the theoretical value 16%. Mechanical retention of nonlinear superposition of light-thermal ageing is significantly lower than single light ageing and thermal ageing for four kinds of HPFs, and it has a significant interaction. Therefore, it is necessary to study effect of light-thermal on high performance fiber. Decay of the HPFs is studied under the condition of low temperature. It is found strength retention of Kevlar(?) fiber is the highest at the low temperature for 24h, above 85%, and it is higher than at 300℃for 77.9%. And mechanical retention of Terlon(?) fiber is 80% at low temperature, it is higher than at 300℃approximately 3%. Mechanical retention of Nomex(?) fibers at low temperature is the lowest, only 58%, while in the heat treatment is relatively high, which was still able to reach above 80%. Mechanical retention of PBO fiber at low temperature is about 70%and it is close to 72% at temperature of 300℃.Comparison of the low temperature, thermal ageing, light ageing and light-thermal ageing has been found, and trends of tensile properties is similar between low temperature and thermal ageing treatment, and the mechanical properties of fibers have different degrees of damage. By light ageing and light-thermal ageing, differences of tensile properties and breakage morphology of PBO fiber and Nomex(?) fibers are obvious. Effect of mechanical properties, structure and composition on HPFs under the condition of low temperature also needs further to be studied and proved. |
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