Studies On The Preparation, Structure And Properties Of New Cotton-Like Copolyesters And Fibers

Cotton possesses many superior properties such as good hygroscopicity, soft handling, easy dyeability and antistatic ability. Therefore, cotton fabrics are comfortable and environmental. Cotton fiber is a kind of crucial material in textile and fashion industries. However, the contradiction between supply and demand of cotton are very serious in China, the demand is much greater than the supply. In order to solve the above contradiction of cotton, China has to import a large number of cotton from other countries every year. Poly(ethylene terephthalate)(PET) has a lot of advantages such as high strength and excellent thermal property. However, disadvantages of PET are also obvious such as rigid handle, low moisture regain and poor dyeability. Nowadays, there are many problems in Chinese polyester industry. For example, the structure of products is too simple, and the supply of polyester is surplus. On this background, studies on the new cotton-like copolyester and its fiber can effectively consume surplus polyester products, and enhance the added value of PET. When modified PET fibers can replace some parts of cotton, it can partly relieve the contradiction between supply and demand of domestic cotton. Therefore, the studies have some practical value and significance in this thesis.On the basis of abundant experiments, the third, fourth and fifth comonomers are eventually chosen. The third comonomer is sodium-5-sulfo-bis-(hydroxyethyl)-isophthalate (SIPE). The fourth is poly(ethylene glycol)(PEG), and the number average molecular weight(Mn) is2000,4000and6000, respectively. The fifth unit is1,2-propanediol (1,2-PDO). The key technological conditions were ascertained in the esterification and polycondensation process such as the catalyst, feed order, feed ratio, temperature, time and vacuum. All samples were successfully synthesized with direct esterification method in the11and101reactor. Their chemical structures were characterized by nuclear magnetic resonance (1H-NMR). Experimental results showed that overwhelming units have been incorporated into molecular chains. The actual molar ratio of all comonomers was basically consistent with the correlative feed ratio. The intrinsic viscosity of all specimens was measured in detail. The results showed that the intrinsic viscosity value of all samples was moderate.Taking advantage of1D wide angle X-ray diffraction (1D WAXD), the crystal structure of copolyesters was explored. After incorporation with PEG and1,2-PDO units, the crystalline structure of copolyesters was nearly identical to the PET homopolymer. It indicated that PEG and1,2-PDO comonomers did not changed the crystal structure. The crystallinity of samples lessened gradually with the increase of PEG and1,2-PDO content. The crystallinity of copolyesters gradually lowered with the decrease of the molecular weight of PEG when the weight percent of PEG was identical (such as10%). Through2D Wide angle X-ray diffraction (2D WAXD) and2D small angle X-ray scattering (2D SAXS), we studied the variation tendency of orientation degree of copolyesters. Before measurements, all specimens were first heated and stretched into different strains along axial direction. At the beginning, the intensity of diffraction and scattering streaks both became stronger when the strain increased. It indicated that the degree of orientation enlarged. Interestingly, after the strain reached a certain value, diffraction and scattering streaks both became weaker when the strain continued to increase. It showed the degree of orientation reduced. The crystal size, crystallinity and long period calculated from ID WAXD and ID SAXS curves owned the similar variation tendency.From differential scanning calorimetry (DSC) measurement, thermal properties were systematically investigated. With the increase of PEG and1,2-PDO percentage, the glass-transition temperature (Tg), the cold-crystallization temperature (Tcc) and the melting temperature (Tm) all decreased gradually. When the weight ratio of PEG was same (such as10%), Tg, Tcc and Tm gradually reduced with the fall of the molecular weight of PEG The non-isothermal kinetics of part of samples was analyzed by Jeziorny method. Results showed that the crystallization process of copolyesters were more complicated than the Avrami model. When the number average molecular weight was identical, as the ratio of PEG6000unit increased, the semicrystalline time (t1/n) decreased, and the cold-crystallization rate improved. The non-isothermal kinetics under different heating rate was studied by Mo method. Tcc of samples enhanced as the heating rate increased. With the increase of crystallinity, the value of F(T) became bigger, but value of a was almost a constant.Thermal stability was investigated in detail by thermogravimetric analysis (TGA) measurement. TGA curves under nitrogen atmosphere were all found to have only one weight-loss stage. While TGA curves under oxygen atmosphere had two weight-loss stages. Under nitrogen and oxygen atmosphere, the degradation temperature all gradually reduced with the increase of PEG and1,2-PDO ratio. The thermal degradation under oxygen atmosphere was easier to occur than that under nitrogen atmosphere. Thermal stability both became worse with decreasing the molecular weight of feed PEG units under nitrogen and oxygen atmosphere. The TGA and dTG curves all seemed to shift towards higher temperature side as the heating rate increased under nitrogen and oxygen atmosphere. Thermal degradation kinetics was studied by Friedman method. Under nitrogen atmosphere, when heating rate is identical (such as10k/min), as the weight ratio of PEG6000unit changed, the curves of Ln(da/dt) vs.1/T and Ln(1-α) vs.1/T both presented linear relationship. Moreover, with the increase of PEG6000content, thermal degradation activation energy (E) became lower, the order of thermal degradation reaction (n) decreased, the thermal degradation rate increased. Under nitrogen atmosphere, for the same sample, as the heating rate changed, the curves of Ln(dα/dt) vs.1/T and Ln(1-α) vs.1/T all exhibited good linear relationship. Thermal degradation kinetics parameters such as E, n and frequency factor (Z) were greatly affected by heating rate. Thermal stability was also analyzed by the intrinsic viscosity variation. When the content of PEG and1,2-PDO were more, the drying temperature was higher and the drying time was longer, then the intrinsic viscosity decreased quickly. It indicated the thermal degradation was easier to happen.By means of contact angle measurements, the hydrophilicity of copolyesters was studied in detail. With the increase of weight ratio of PEG and molar ratio of1,2-PDO, all contact angle values gradually became smaller, and the hydrophilicity of samples were better. When the weight percent of PEG was identical (such as10%), the values of contact angle were detected to gradually reduce as the number average molecular weight of feed PEG units lowered, indicating that the hydrophilicity of samples was enhanced.Some copolyester chips were spun into cotton-like fibers by melt spinning method. Spinning technological parameters were determined such as drying temperature, spinning temperature and rate, draw ratio. By measuring reel and weight method, linear density of fibers was calculated. The crystallinity of fibers was studied by wide angle X-ray diffraction (WAXD). Results showed that the crystallinity decreased with the increase of content of1,2-PDO. By means of WAXD and acoustic wave propagation method, the degree of orientation of fibers was explored. It was found that the orientation factor (fs) and degree of orientation both lowered as the1,2-PDO content increased. The mechanical properties of fibers were measured by electronic single yarn strength tester. Experiments indicated that when molar ratio of1,2-PDO unit increased, The breaking tenacity and initial modulus were gradually reduced, yet elongation at break enhanced. The moisture regain of cotton-like fibers was tested. Results exhibited the increase of1,2-PDO content improved their moisture regain and hygroscopicity. The boiling water shrinkage of fibers was measured. The results indicated that the shrinkage increased with the increase of content of1,2-PDO. Making use of contact cool and warm feeling tester, the contact comfort of cotton-like fabrics was analyzed. It was found the Qmax value of cotton-like fabrics was smaller than that of PET fabrics. Moreover, the Qmax value of cotton-like fabrics gradually decreased when the molar ratio of1,2-PDO unit enhanced. The Results indicated the contact comfort of cotton-like fabrics became better with the incorporation of1,2-PDO comonomer.