Studies On The Preparation, Structure & Properties Of Biodegradable Poly (Butylene Succinate-co-butylene Terephthalate)s (PBST) Fibers

Synthetic fibers have afforded a significant improvement in the quality of life and availability of products for consumption. However, the wide application of these synthetic fibers causes some problems:firstly, the sources of raw materials, synthetic fibers derived from oil result in the problem of limited and non-renewable oil resources which cannot afford our demand; secondly, the problem of fiber waste treatment, most of these synthetic fibers are non-degradable and thus exacerbate the existing ecological and environmental problems. To solve these problems, some effective methods have been carried out. Firstly, material recycling, high purification technical is needed, and the recycled materials generally possess poor performance and thus are used as middle or low class products. Furthermore, second pollution may be caused during recycling process. Secondly, develop biodegradable fibers to solve the problems both from source shortage and environmental pollution, which is the new trend for the international development of synthetic fibers.Biodegradable poly(butylene succinate-co-butylene terephthalate) (PBST), which was demonstrated to possess good thermal and mechanical properties, has attracted the attention of researchers. The traditional preparation method was to use the more mature synthesis method of transesterification. The high costs of raw materials (dimethyl esters) and complex separation of by-products (eg. methanol and tetrahydrofuran) confined their bulk production. In the context, the route of direct esterification and polycondensation was adopted to synthesize PBST copolyesters from the starting materials of terephthalic acid (TPA), succinic acid (SA) and 1,4-butanediol (BD). Titanium tetraisopropoxide (TTiPO) was selected as the main catalyst, and the combination of TTiPO and catalyst promoter can significantly accelerate the reaction rate and increase the esterification rate. The complex reaction was determined to be the dominant catalytic reaction mechanism, similar to that of poly(butylene terephthalate) (PBT). The reaction process can be divided into two steps, esterification and polycondensation. The proper feed ratio of diol to diacid was determined to be 1.5, the esterification temperature, pressure and time were respectively determined to be 190-200℃,80-95Kpa and 2h. Then the polycondensation temperature was controlled at 235-240℃, and the reaction time under high vacuum was controlled at 2-3h. 1H NMR proved the prepared products of the PBST copolyesters. GPC results showed that the weight average molecular weights of the PBST copolyesters were in range of 9.71-13.28, and degree of distribution ranged from 1.92-2.40.From DSC and TGA measurements, the effects of catalyst system on the thermal properties of PBST copolyesters were investigated. The results showed that PBST catalyzed via complex catalyst exhibited relatively higher melting temperature (Tm) and higher thermal degradation temperature (Td). Furthermore, the import of stabilizers enhanced the thermal stability of PBST to some extent, triphenyl phosphate (TPP) exhibited better effect, the Td reached 389.1℃. Furthermore, the tensile test results presented that the PBST copolyesters catalyzed via complex catalyst had higher breaking strength and higher elongation at break. As well, higher initial modulus, higher breaking strength and higher elongation at break were obtained with increasing the content of BT units. The results of DMA analysis showed that with increasing the content of BT units, the glass transition temperature (Tg) increased; the storage modulus (E') tended to heighten indicating the more prominent elasticity; and the peak of loss modulus (E") shifted to higher temperature, indicating the copolyester with higher BT units content had the more prominent viscosity.The spherulitic growth kinetics of PBST copolyester was investigated at a wide crystallization temperature from 70 to 170℃via POM. The crystal lamellar thickness at different isothermal temperatures was analyzed via SAXS. The results presented that the overall crystal growth rate of PBST was normal distribution. The maximum growth rate was obtained at 90℃, then the spherulite growth rate was slowed and the lamellar thickness was increased with the increasing isothermal crystallization temperature. The isothermal crystallization kinetics of PBST-70 (the content of BT units was 70mol%) was investigated based on the Lauritzen-Hoffman secondary nucleation theory. The results exhibited that the classical regimeⅠ→Ⅱand regimeⅡ→Ⅲtransitions occurred at the temperatures of 150℃and 110℃respectively. And the empirical Hoffman universal values of U*=1500cal/mol and T∞=Tg-30K were fit for the secondary nucleation theoretical analysis of PBST copolyester. In addition, melting temperature and mechanical properties of PBST copolyester were heightened with the increasing isothermal crystallization temperature.The PBST fully drawn yarns (FDY) were obtained by one-step melt-spun method. The results showed that suitable spinning temperature of PBST copolyester was determined to be 240-260℃, and the spinning process significantly affected the aggregation structure, tensile properties, elastic properties and boiling water shrinkage rate of PBST fibers. The PBST draw textured yarn (DTY) was obtained via false-twist texturing process. The investigation results presented that the draw ratio, deformation temperature and the heat setting process influenced the tensile properties and boiling water shrinkage of PBST fibers. And the curly deep, plump appearance of PBST DTY fibers was obtained when D/Y ratio was set at about 2.0.Furthermore, the tensile and cyclic stretch elastic recovery properties of PBST fibers were investigated with PBT fibers as a comparison sample. The strain rate, constant elongation and cyclic stretch times all affect the elastic recovery behaviors of PBST fibers. Better stretch elastic properties of PBST fibers were found compared with that of PBT fibers under the same test conditions, and more obvious differences between them were found with increasing the constant elongation. Meanwhile, the stress relaxation and creep properties PBST fibers were superior to that of PBT fibers, which were attributed to that the combination of soft BS unit and hard BT unit of PBST fibers made contribution to obtain better dimensional stability. The mechanical model was established to analyze the viscoelastic property of PBST fibers for the purpose of describing the relaxation and creep behaviors of PBST fibers more intuitively. The results showed that the simulation worked well, and the correlation coefficient can reached above 0.98. The stretch elastic recovery mechanism of PBST fibers can be explained through the analysis of crystal transition of PBST fibers during drawing process. PBST fibers were found to undergo PBT-like crystal form transition from a-form toβ-form crystal structure under tension load. The crystal transition was found to occur in a higher strain region compared with that of PBT fibers. As to PBST fibers, the a-form still exists even at the tensile strain of 25%, which endowed PBST fibers with higher elastic recoverability compared with that of PBT fibers.The hydrolysis behavior of PBST fibers was conducted at conditions of different hydrolysis temperatures and pH values. According to the results, it could be observed that more significant hydrolysis behavior of PBST fibers was found as the extended hydrolysis time, the surface changes of PBST fibers before and after hydrolytic degradation. Originally, the surface of PBST fibers was smooth, while the surfaces became rough, more and more spots appeared as the extended hydrolysis time, showing that the hydrolysis process started from the fiber surface. The hydrolysis temperature significantly affected the hydrolysis behavior of PBST fibers. The weight average molecular weights of the PBST fibers decreased, while the content of BT units increased with increasing the hydrolysis temperature. Moreover, pH value also affected the hydrolysis behavior of PBST fibers, and the hydrolysis rate exhibited that alkaline>acid>neutral, which was due to the reason that different hydrolysis mechanism happened under different pH values. The hydrolysis temperature, hydrolysis time and pH value all affected the tensile properties of PBST fibers. The results showed that the breaking strength, elongation at break and initial modulus decreased with increasing the hydrolysis temperature and time. In different conditions of pH values, the drop scope of tensile properties of PBST fibers presented that alkaline> acidic> neutral.