The Structure And Properties Of PET Tire Cord Yarns In Low Strain Region

PET industrial yarn not only has a good chemical stability as common yarns but also has excellent mechanical performance such as high breaking tenacity, high modulus, and good impact resistance. PET industrial yarns are mainly used as tire cords. It has been showed that the modulus of PET fibers vary with the increase of strain. The minimum of the modulus occurs in the strain range from 2% to 5% which is the range of the tire cords strain in application. So the modulus-strain curve is important to describe the mechanical properties of PET industrial fibers.This paper analyzes the mechanical performances of PET industrial yam especially the modulus-strain curves in 2~5% strain range. Combining with the changes of fine structure of PET industrial yarn during stretch the paper analyzes the reason of the declining of modulus in this strain range and on the base of the structure research, spin PET industrial yarn.The paper studies the index of the mechanical properties of PET industrial yarn for tire cord. Firstly, it evaluates the index such as stress-strain curve, initial modulus, modulus-strain curve and determines the corresponding indexes which sufBciently reflect the mechanical performance of PET fiber for tire cord and testes the modulus-strain curves under difiFerent temperature and the dynamic viscoelasticity of the strain range 2%-5%.With the basis of the modulus-strain curves, we test the crystallinity, tie-taut molecular and orientation degree, etc. We apply the high viscosity PET chip on the spin machine to get the PET industrial yarn through adjusting the spinning speed, and drawing ratio to control the fine structure of the yarn.The modulus-strain curve even fully reflects the mechanical properties of PETindustrial yarn. Fitting the stress and strain data gets the stress-strain curve and thebest fitting exponent is four. Derivative the stress-strain curve gets the modulus-strain curve. The modulus-strain curves of different PET industrial yarn are two peakscurves under the testing temperature from 25 °C to 200 "C.But the peak value and the minimum value of the modulus curves are different.The dimensional stability PET industrial yarn have higher degree of crystallinity and orientation, higher structure regularity, continues crystal, and lower amorphous orientation, higher tie-taut molecular, lower coil molecular, better heat stability.In order to attain the dimensional stability fiber and increase the modulus of the lower strain range, the following conditions are fulfilled.1) Fiber has higher degree of crystallinity, orientation, and higher structure regularity. The crystal regions cross each other and higher tie-taut molecular in amorphous region.2) Higher cross-linking molecular and higher inter-stress make the fiber has higher shrinking temperature and lower shrinkage.PA industrial yarn, high tenacity PE, Ka, carton fiber and high tenacity rayon have their own modulus-strain curves. When the crimple molecules uncoil, the eonformer of molecules transit from gauche to trans. After loaded, the glycol groups of the PET molecules are elongated and distorted. The entanglements networks and the cross-links between molecules such as hydrogen bonds are broke off. If the load is moved out, the molecules come back instantly and the strain is about 0.1%. The denser the network is the higher force to destroy the network is. the crimpy molecule's uncoiling leads to the declining of the modulus. This process is accompanied by the conformation transitions. The ester groups at the aromatic rings of PET molecule have a mutual cis or gauche positioning. The trans eonformer is related to the straight parts of the crystalline region and the gauche eonformer is related to the bended parts of amorphous region. When the crimple molecules uncoil, the eonformer of molecules transit from gauche to tran. Loaded continually, the bone chains of the straight molecules begin to tighten and the strain causes the stretching of covalent bond angles. The crystalline regions start to slip until some molecules rupture is initiated in the amorphous regions. Since the chain modulus of the taut-tie molecules is relatively high, the tensile modulus of the fiber starts to increase.Because of the length distribution of molecule, the molecules cannot bear the loadat the same time and the modulus increases gradually. A prerequisite for the tie molecules to increase the modulus when becoming taut is that they are constrained by crystals. The scissions of chains cause the modulus to pass through the second maximum. Beyond the second modulus maximum, the broken chain ends recoil and the amount of gauche increases again.Dimensional PET fiber is spun by two-step method, high viscosity chip and high temperature high drawing ratio. The parameters of two-step method: spinning-speed is 3000m/min, drawing temperature is 210°C, and drawing ratio is 2.2. At the same drawing condition, the yarn which is pre-drawn has better mechanical properties than the yarn which is not pre-drawn. With the increase of drawing temperature ,the tenacity, initial modulus, Lase-5, the minimum modulus increase and the breaking elongation and shrinkage decline.Stable crystal forming under the higher temperature and higher drawing ratio makes the fiber come into the even crystal and keep the orientation in the amorphous region. So the tie-taut molecular are formed between two crystal regions and the minimum value of modulus increase.