| Biodegradable poly(butylene succinate-co-butylene terephthalate) (PBST) copolyester is now attracting much more attention owing to its properties. It combined the advantages of aliphatic and aromatic polyester, encompassing biodegradability and high performance properties; At the same time it can reduce the oil consumption because the raw materials of 1,4-butanediol (BDO) and succinic acid (SA) can be obtained by the method of bio-ferment, so it has a good prospect in application. But all the research now is focus on the product performance, including its synthesis method, structure, characters, but no the kinetic model for the entire process has been reported. So our study is aiming at studying the reaction kinetics on the complex reaction mechanism in the synthesis process to get some basic data. In our work, first we proposed a two-esterification reactor in parallel esterification process, then studied reaction kinetics and model of the esterification and poly condensation, and at last synthesized PBST with different composition and studied its composition, structure, thermal and mechanical properties..First of all, did the experiment of SA/BDO and TPA/BDO esterification catalyzed by TBT. The volume of the reaction system is changed liner with the reaction degree of the COOH (TPA increased and SA decreased); And Comparing the water and THF amounts with the initial BDO amount, it can be seen that over 80-85% BDO were converted to water and about 3-6% to THF during the esterification stage.A theoretical mathematical model for the esterification reaction at the presence of metal catalyst TBT has been developed and applied successfully in the simulation of all experimental data, which include two esterification reaction(k1=2k2), one polycondensation(k3) and two side reations(k4,k5). The activation energy E1 in SA/BDO system (59.5kJ·mol-1) is lower than in TPA/BDO system(82.2 kJ·mol-1). E1 E3 (59.5,89.6 kJ·mol-1) in SA/BDO system in our study is a little higher than the same reaction (59.5,89.6 kJ·mol-1) as previously reported in literature. The effects of reaction temperature, diol/diacid ratio and catalyst amounts on the esterification kinetics of SA/BDO are simulated and the differences between the two esterificaiton systems are compared. Higher reaction temperature results in higherpolymerization degree of the oligomers and THF formation; Hihger diol/diacid molar ratio leads to higher esterification rate and THF formation but lower polymerization degree; The lower rate constants and heterogeneity make TPA/BDO esterification slower than SA/BDO.The intrinsic kinetics of the PBT, PBS polycondensation and PBST copolycondensation reaction has been studied using thermogravimetric analysis. It appeared that the data could be well described using a second-order polycondensation reaction at this catalyst concentration. The rate constants for PBT, PBS and PBST polycondensation reaction are kT=3.15E5·e-63350/RT,kS=8.05E4·e-62200/RT, kST=2.05E4·e-48770/RT respectively. The activation energy for PBT, PBS polycondensation were found to be 63.4kJ/mol,62.2kJ/mol respectively, which are lower comparing with the data reported before (PBS 85.7kJ/moD.N.Bikiaris,2008; PBT 85.4kJ/mol, P.J.Darda 2005).At last, synthesized PBST copolyester with different ratio of TPA/SA by the optimized condition, and studied the composition, structure and other characters. The composition is agreed with the ratio of material. The degree of randomness are all below one, the sequence length of butylene terephthalate units is less than 3, which is under the degradation condition, when the SA content is in the rang of 50-80%. When the TPA content increased the melting point (Tm) and the heat of melting is first decreased and then increased, but the glass transition temperature (Tg) is always increased. And there is no Tm found when the SA is 70% in composition. The tensile result shows that PBST has a second shape change process; and the higher PBT units content is helpful to increase the strain at break (%). |
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