关键词: 阳离子可染聚酯, 毛细管流变, 旋转流变, 流变性能, 离子聚集

Abstract: Polyethylene terephthalate (PET) has low production cost, excellent mechanical properties, high thermal stability and excellent corrosion resistance. The polyester fiber formed by its processing is one of the world's largest synthetic fiber varieties, which is widely used in the field of textiles. The symmetrical molecular structure of PET and the absence of chemically active sites in the molecular chain lead to its easy crystallization and high crystallinity, poor hygroscopicity, and difficulty in dyeing. Therefore, the dyeing modification of PET to meet the demand for new textile products is in line with the development of the textile industry. Cationic dyeable polyester (CDP) fiber is the earliest successful development and currently the largest industrial production of modified species among the dyeing modifications of PET. However, in the actual production process, the introduction of sodium-5-sulfo-bis-(hydroxyethyl)-isophthalate (SIPE) makes the coking more severe in the pipeline, reflecting that the macroscopic flow behavior of the melt changes significantly after the introduction of SIPE.
In order to explore the changes of rheological properties of CDP melt, we analyzed the shear viscosity and non-Newtonian index of PET and CDP with similar intrinsic viscosities at the same die diameter capillary rheometer and rotational rheometer tests. On this basis, the viscous flow activation energy and structural viscosity index of the melt were obtained by analytical calculations. The variation of melt viscosity at different die diameters has also been studied. Subsequently, frequency scans and time scans were performed to investigate the trend of CDP melt viscosity with time. It is found that the ionic aggregation effect of strong polar sulfonic acid groups will form ionic aggregates which will increase the physical cross-linked structure inside the CDP melt, resulting in a significantly higher viscosity and structural viscosity index than the PET melt under similar characteristic viscosity. The formation and depolymerization of ionic aggregates are affected by temperature and shear, which makes CDP melts more sensitive to changes in temperature and shear rate than PET melts; the more relaxed the melt molecular chain is under large diameter, the lower the viscous flow activation energy and the non-Newtonian index is closer to 1; the heat generated by viscous dissipation of the melt increases at small diameter and the shear viscosity decreases; the melt viscosity of CDP tends to increase with the increase of heating time.
In the actual CDP production process, the change of melt viscosity will have a large impact on the production, and the appropriate melt temperature, conveying flow rate and extrusion rate should be selected with the analysis of rheological properties. The results of this study are conducive to analyzing the essential causes of coking caused by pipeline blockage due to viscosity changes during the pipeline transport of melt and finding the solution.

Key words: cationic dyeable polyester, capillary rheology, rotational rheology, rheological properties, ion aggregation