| PLA is a kind of thermoplastic aliphatic polyester produced from renewable biomass resources.It is non-toxic,non-irritating and has a good biodegradability,biocompatibility and processability.Therefore,PLA can be used as a potential alternative to petroleum-based polymers products,and its products are widely used in the fields of packaging materials,textiles and clothing,agriculture and so on.Currently,the industrial preparation of PLA with a higher molecular weight is generally carried out by ring-opening polymerization of lactide by using kettle or tower reactor as a reactor.However,the polymerization time is usually over 20 h,and thus the problem of production cost caused by low productivity still restricts the extensive application of PLA products.Compared with the other polymerization methods,the reactive extrusion method by using a twin-screw has many advantages,such as simple production equipment and process,fast reaction rate,high production efficiency and low production cost.And the obtained PLA has good mechanical property,processability and so on.Till now,there were some domestic patents in this area and our group also carried out a preliminary study of polymerization by using stannous octoate and triphenylphosphine as a catalytic system before.However,further studies are still needed.Therefore,based on the previous studies,this thesis made a further research in this area.Firstly,a L-lactide sample with favorable performance was chosen as main raw material of polymerization after comparing the performance of two L-lactide samples.Then by using L-lactide as raw material and twin-screw extruder as reactor,the influences of the monomer purity,the kinds and the content of catalyst systerms,polymerization time,polymerization temperature,the screw speed of the extruder and other factors on the structures and properties of the polymerization product were studied.The results showed that the relatively appropriate polymerization conditions were as follows:2-ethylhexanoic acid tin(II)salt was selected as the main catalyst and 1,dodecanol was selected as the cocatalyst,the polymerization temperature was 170 ?,the molar ratio of monomer/catalyst was 3000:1,the screw speed of the extruder was 110 rpm and the polymerization time was 2.0 h.The PLA polymer with a viscosity average relative molecular weight of about 53,000 and an optical purity of 96%could be obtained under the above conditions.The structure and thermodynamic property of the polymer were further characterized by FT-IR,1H-NMR,DSC,XRD,etc.Both FT-IR and 'H-NMR results verified that:under above appropriate conditions the polymer synthesized was Poly(L-lactic acid)(PLLA);DSC and XRD test results indicated that the PLLA obtained by ring-opening polymerization of L-lactide is a crystalline material with a melting-point at 169.0 ? and a mixed a' and a crystal phase.TG/DTG results showed that the maximum decomposition temperature(MDT)of the of PLLA at 368.0 ?and 5%weight loss temperature at 310.0 ?.Based on the above results,the kinetics of L-lactide ring-opening polymerization was further studied.The results showed that with the extension of the polymerization time,the monomer conversion rate first increased and then tended towards stabilization with an equilibrium conversion rate of 96%,while the viscosity average relative molecular weight of the PLLA first increased and then decreased.The reaction kinetics equation indicted that the polymerization showed first-order kinetics for both catalyst concentration and L-lactide monomer concentration.The apparent activation energy(?Ea)and frequency factor(A)were 39.8 kJ/mol and 1.82×107,respectively.The research results of this thesis indicated that by using L-lactide as the raw material,stannous octoate as catalyst and 1-dodecanol as cocatalyst,the PLLA polymer with a certain viscosity average relative molecular weight can be obtained by ring-opening polymerization rapidly using a twin-screw extruder. |
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