| Poly(ethylene terephthalate)(PET), one of widely used commodity synthetic polymers with huge production, plays an indispensable role in our life. Therefore, the recycling of PET has found its continuously growing popularity due to economic and ecological considerations. Chemical recycling based on depolymerization and repolycondensation, is a major recycling method of PET, which can achieve the high quality and value-added products. Meanwhile, glycolysis of PET with ethylene glycol(EG) is one of the most favorable chemical recycling processes, because it requires mild reaction conditions, shorter process, and can be implement in conventional production equipments of PET. Therefore, in this thesis, several issues occurring in the recycling of PET via glycolysis and repolycondensation have been investigated.Although the conventional catalysts of PET glycolysis are effective, the residual of these catalysts will degrade the quality of regenerated PET(r PET) owing to their undesired catalytic activities for repolycondensation, which means high cost of catalyst separation is required. In this thesis, sodium titanium tris(glycolate) was synthesized and applied as catalyst for glycolysis and repolycondensation. Based on the results of comparative experiments, in glycolysis, sodium titanium tris(glycolate) presented higher activity than sodium carbonate and tetrabutyl titanate did, and only a little lower activity than zinc acetate did. Meanwhile, the yield of BHET was 85%, which was very close to the yield when zinc acetate was used as catalyst. On the other hand, in repolycondensation, sodium titanium tris(glycolate) also presented good catalytic activity. Most importantly, it showed much lower catalytic activity for thermal degradation of PET than zinc acetate and tetrabutyl titanate did. Therefore, sodium titanium tris(glycolate) can be regarded as a suitable catalyst for the recycling of PET via glycolysis and repolycondensation. Meanwhile, the corresponding catalytic mechanism was also investigated by Fourier transform infrared spectroscopy(FTIR) and nuclear magnetic resonance(NMR). The results showed that two catalytic mechanisms(1.by forming the metal alcoholate to accelerate the nucleophile; 2.by ligand-exchange reaction) were all presented in glycolysis catalyzed by sodium titanium tris(glycolate). Consequently, the corresponding mutual promotion mechanism was also proposed.The glycolysis of PET with ethylene glycol is usually carried out under the boiling point of EG at atmospheric pressure as a heterogeneous reaction process. While the corresponding kinetics was ill-fitted by the conventional model. Therefore, in this thesis, the heterogeneous glycolysis of PET catalyzed by sodium titanium tris(glycolate) has been investigated. Based on the evolution of solid and liquid phase changing during the reaction, two main reasons which were responsible for ill-fitting by conventional models were identified as follow: 1) the conversion in solid phase has been accelerated by depolymerized products; 2) glycolysis will reach an equilibrium stage at last. Based on these two factors, by describing the reaction as a transfer of PET repeat unit form solid phase into soluble oligomer, dimer and BHET, a more accurate kinetic model for heterogamous glycolysis of PET was built up, which could well describe the three stages of the glycolysis: 1) lower-rate stage at the beginning of the reaction; 2) acceleration stage in the middle of reaction; 3) the equilibrium stage at the end. The corresponding activation energy and enthalpy of glycolysis were 90.30 k J·mol-1 and 22.17 k J·mol-1 respectively.The amount of diethylene glycol(DEG, 2-(2-hydroxy-ethoxy)-ethanol) contained in r PET produced through glycolysis was relatively higher than that in the original PET. This problem is difficult to solve because the formation of DEG in the glycolysis is inevitable. Aiming for making the best of the advantages and bypassing the disadvantages, in this thesis, by using DEG and adipate(AA) as raw materials to modify PET, a series of poly(ethylene terephthalate-co-ethylene adipate-co-ethyleneoxyethylene terephthalate-co-ethyleneoxyethylene adipate)(PETDA) with low-melting point was synthesized and characterized. The chemical structures of these copolymers were identified by FTIR and 1H-NMR. The degrees of randomness of these copolymers were ranging from 0.82~0.90. When the DEG feeding was higher than 25%(corresponding to the total amount of diol), the amount of DEG involved in copolycondensation was less than feeding. The thermal properties of these copolymers were characterized by thermal gravity analysis(TG), differential scanning calorimetry(DSC) and optical microscope. The results showed that the onset decomposition temperature(Tonset), crystallization ability, melting point(Tm) and glass transition temperature(Tg) were all decreased with the copolycondensation ratio increased, the Tonset were ranging from 372~379 ℃ and changing slightly when the copolymer ratio changed. The crystallization abilities of copolymers will disappear when the copolycondensation ratio was higher than 30%. Based on the equilibrium melting temperatures of copolyesters measured by the Hoffman-Weeks method, the melting point depression of this series copolyester was well-fitted by Baur model. Meanwhile, considering the similarity of crystal structure between copolymers and PET revealed by X-ray diffraction, the crystals of cosegments of these copolyesters could be treated as a pseudo-eutectic system. The brittle-ductile transition happened while the ratio of copolycondensation increased, and the copolymer with copolycondensation ratio at 25~40% presented obviously elastic resilience.In order to prepare the thermal bonding sheath-core conjugated fiber by using PETDA as sheath and PET(textile grade) as core, the copolymers have been optimized according to the thermal bonding strength between copolymers and PET fabric. P25(copolymer with copolycondensation ratio at 25%) was selected. The rheology of P25 and PET melts were investigated by capillary rheometer. The results showed that the non-Newtonian index and the structural index of P25 melt were more sensitive to the temperature and shear rate changes compared to the case of PET. The isothermal crystallization kinetics of P25 was investigated by DSC. the Avrami indexes were approximately 2, the maximum isothermal crystallization rate could be obtained at 110℃, and the corresponding half-crystallization time is 18.65 min. Based on the rheological results and the calculated values of shearing rates at sheath-core conjugated spinning nozzle, the ideal temperature ranges of conjugated spinning were speculated and used as guidance for parameter setting. Meanwhile, the heat treatment of fiber was proceeded under the guidance of isothermal kinetics results. When the ratio of sheath-core at 3:7, the optimal property of conjugated fiber was achieved, with fiber strength at 3.09 c N·dtex-1 and elongation at break at 22.35%. |
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