Abstract:

Objective The continuous expansion of the application field of polyester industrial fiber puts forward more detailed requirements for its performance, and the relationship between process-structure-performance needs to be further clarified. In order to explore the intrinsic structural factors of the differences in the application fields of high-tenacity polyester industrial yarns obtained at different heat-setting temperatures, the structure and properties of three high-tenacity polyester industrial yarns were compared.

Method Synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) were adopted to study its multi-scale microstructure. Establish a multi-level structure analysis method from macro to micro was established, and the influences of different heat-setting processes on the structure and properties of high-tenacity industrial yarns were clarified.

Results The fiber spinning process differences between the three high-tenacity polyester industrial yarns were reflected in the difference of heat-setting temperatures (Tab. 1). Compared with high-tenacity medium-shrinkage (HTMS) and high-tenacity (HT) polyester industrial yarns, high-tenacity low-elongation (HTLE) was shown to have lower heat-setting temperature, resulting in higher amorphous orientation, lower crystallinity, smaller crystallite size, smaller long period and larger tilting angle of crystalline lamellae (Tab. 4 and Tab. 5). Because the microstructures with high crystallinity and high orientation are formed under the condition of high drafting ratio, the breaking strength of the three industrial yarns are relatively high, and the differences are not obvious(Tab. 2). The mechanical properties are different in elongation at break, initial modulus, elongation at a specific tenacity of 4.0 cN/dtex (Easl-4) and tenacity at a specific elongation of 5% (Lase-5). HTLE polyester has the smallest elongation at break, the largest initial modulus, the largest thermal shrinkage, the worst dimensional stability, and the highest α transition temperature. HTLE has the highest sound velocity orientation and small deformation during stretching, and thus has the lowest elongation at break. The amorphous orientation of the fibers is the key structural factor determining the elongation at break due to the small difference in the crystallite orientation of the three industrial yarns. The initial modulus appears in the first stage of the stretching process, which is mainly related to the amorphous region. With the increase of the amorphous orientation, the initial modulus also increases. Therefore, the amorphous orientation of HTLE is the largest, and its initial modulus is the largest. The heat-setting temperature of HTLE is low, the fiber shrinkage is small, the molecular orientation is large, and the thermal shrinkage is maximum (Tab. 3). In addition, crystallization will form a cross-linking effect, limiting the movement of molecular chains, and also has an impact on the thermal shrinkage performance of the fiber, therefore, crystallinity of HTLE is low, the crystallite size is small, and the thermal shrinkage is the largest. The dimensional stability refers to the sum of the Easl-4 and thermal shrinkage rate, and the smaller the sum is, the better the dimensional stability is. In addition, the tilting angle of crystalline lamellae also has a certain influence. When angle is small, it can be considered that the fiber has a regular structure and good dimensional stability. HTMS has a smaller thermal shrinkage rate and a smaller tilting angle of crystalline lamellae, and thus has good dimensional stability (Tab. 3). The higher the glass transition temperature is, the higher the temperature at which the molecular chains in the amorphous region begin to have thermal motion, the larger the chain binding, the smaller the activity capacity. HTLE has the highest amorphous orientation, which limits the movement of molecular chain, resulting in the highest T_g (Fig. 3).

Conclusion The heat-setting temperature mainly affects the amorphous orientation and the lamellar structure of polyester industrial yarns. Compared with HTMS and HT, HTLE has the lowest heat-setting temperature, and the stretched amorphous molecular chains produced by high draw ratio didn't enter the crystal lattice to form crystallization, and occurred a small recovery at low heat-setting temperatures, which causes HTLE industrial yarns to show the structural characteristics of high amorphous orientation, low crystallinity, small crystallite size and large tilting angle of crystalline lamellae, resulting in the lowest ultimate elongation, the highest initial modulus, the worst dimensional stability.

Key words: shigh-tenacity polyester industrial fiber, crystalline structure, lamellar structure, small-angle X-ray scattering, wide-angle X-ray diffraction, mechanical property, heat-setting process