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Synchrotron radiation wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) were performed to study the structures of four typical types of poly(ethylene terephthalate) (PET) industrial yarns. Three-dimensional structural models of the yarns and comprehensive insights into the process-structure-property relationships were gained. High spinning speed, low draw ratio, and high heat-setting temperatures lead to HMLS yarns with high crystallinity, high amorphous orientation, densely packed lamellar stacks, and a small tilting angle of crystalline lamellae. High draw ratio tends to result in PET industrial yarns with large long period and a large tilting angle of lamellae. Heat-setting process has a significant influence on the amorphous orientation and crystalline structures, such as crystallinity, crystallite size, as well as crystal grain number. Compared with other structure characteristics, amorphous orientation plays a more important role in determining the tenacity, initial modulus, part load elongation, ultimate elongation, as well as shrinkage of PET industrial yarns. The crystal grain number seems to have an effect on the initial modulus, while the long period influences the elongation of the yarns to some extent. In addition, the small tilting angle of crystalline lamellae may relate to the dimensional stability of PET yarns.

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