| Poly(ethylene terephthalate)(PET) industrial yarns are commercially availablefor wide range of applications viz. textile reinforcement in passenger radial tires,conveyor belts, v-belts, mechanical rubber goods, seat belts, coated fabrics,geo-textiles, etc, due to its high strength, high modulus, low elongation, good heatresistance, impact resistance and fatigue resistance. However, with the rapid changeof the global industry, traditional PET industrial yarns have been unable to meet theneeds of the development. New types of yarns with high performance and added value,such as heavy denier, fine denier, high modulus and low shrinkage, high tenacity andsuper low shrinkage, antibacterial, waterproof and flame retardant have become a keydevelopment direction of PET enterprises. Therefore, having a good command of theprocess-structure-property relationships is of great significance. In this paper,synchrotron radiation small angle X-ray scattering (SAXS) and wide angle X-raydiffraction (WAXD) were carried out to study the structural differences of four typicalPET industrial yarns as well as five types of HMLS PET industrial yarns.Time-resolved SAXS and WAXD were performed to investigate the structuralevolution of HMLS yarns at different temperatures during tensile deformation. Inaddition, time-resolved SAXS was carried out to study the crystallinzation behavior ofPET under shearing during isothermal crystallization. Three-dimensional structuralmodels of PET industrial yarns and comprehensive understanding ofprocess-structure-property relationships were gained.High modulus and low shrinkage (HMLS), high tenacity (HT), low shrinkage (LS) and super low shrinkage (SLS) PET industrial yarns are the most widely usedtypes. It was found that high spinning speed, low draw ratio and high heat-settingtemperatures lead to HMLS yarns with high crystallinity, high amorphous orientation,small long period, densely packed lamellar stacks and a small tiling angle ofcrystalline lamellae. High draw ratio tends to result in HT, LS and SLS yarns withlarge long period and a large tilting angle. Heat-setting temperatures have a significantinfluence on the amorphous orientation and crystalline structures such as crystallinity,crystallite size as well as crystal grain number. Compared with other structuralcharacteristics, amorphous orientation plays a more important role in determining thetenacity, initial modulus, part load elongation, ultimate elongation and shrinkage. Thecrystal grain number seems to have an effect on the initial modulus, while the longperiod influences the elongation of PET yarns to some extent. In addition, the tiltingangle of crystalline lamellae might relate to the dimensional stability of PET yarns.The results of different types of HMLS yarns suggest that crystallinity affects thetenacity and shrinkage. A low value of crystallinity indicate more amorphous phase inthe yarns, which is sure to decrease the tenacity and increase the shrinkage.Consequently, high crystallinity and high amorphous orientation are essential for PETyarns to keep both high tenacity and low shrinkage. Moreover, it was found that PETyarns with a small tilting angle of crystalline lamellae have a good dimensionalstability, which is consistent with our previous conclusion.Tensile deformation at different temperatures of HMLS yarns enabled us to haveaccess to simulate the drawing and heat-setting process. Time-resolved WAXD andSAXS results indicate that crystallinity and crystallite size along equator decreasedwhile the long period, thickness of crystalline and amorphous domain increased withthe increasing strain of HMLS yarns at25oC，95oC，125oC. However, at200oC thecrystallite size associated to (-110) and (100) reflections increased to some extent withthe increasing strain, while the crystallinity and crystallite of (100) reflectiondecreased. It is evident that both crystalline and amorphous phase were involved andfiber structure became loosely packed during tensile deformation. Based on ourprevious results, high spinning speed and low draw ratio should be adopted in order toproduce PET yanrs with compactly packed structures.Time-resolved SAXS measurements of PET indicate that shear inducedorientation of crystalline lamellae, which can be confirmed from the anisotropictwo-dimensional SAXS patterns of PET chips. In addition, the crystallization of PETunder shearing appeared earlier than that of PET without shearing indicating that shearing promotes crystallization rate. In the course of manufacturing, improvingspinning speed means larger shearing effect by the spinneret, leading to the increaseof orientation and crystallization of PET. |
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