| PLA(polylactic acid) is one of the most promising biodegradable man-made polymers, They have been employed as absorbable surgical suture, for internal bone fracture fixation, as scaffold for extracorporal tissue culture and as controlled drug-delivery systems in medicine and pharmaceutical technology since these polymers are broken down in the body to from natural metabolic products. Lactide is the intermediate for the preparation of PLA from lactic acid, but its low conversion is a great hamper to apply PLA extensively in such area as biomedicine and environmental protection.A novel way is introduced to improve lactide preparation in a relatively inexpensive condition but acting more efficiently, which is based on enhancing the reaction's catalyst activity while decreasing the reacting temperature and reducing the whole reacting system's viscosity. A composite oxide material, Zinc-Lanthanum-titanate, was prepared using a sol-gel method. As a catalyst for lactide prepared, this composite exhibited an excellent quality of catalytic property in dehydrate reaction including lowering the reacting temperature and viscosity simultaneously. The intermediate material, lactide, is then filtered and purified with the help of solvents, and it can be used to prepare PLA in a preferably predict results.At first, based on the character and catalyzing theory of catalyst, the lanthanum(a lanthanon) and titanium(a transition tantalum), both have abundant empty orbits, were selected, while Zinc is taken as an active element of catalyst, and the method of sol-gel is adopted to prepare the composite oxide, Zinc-Lanthanum-Titanate. The catalyzing activity of this composite roots in replacement action of vary valence metal ion in the crystal lattice, and new active centers are formed by this kind of crystal lattice defects. This composite is then used as the catalyst to synthesize lactide from lactic acid, and more, the intermediate product, lactide, is synthesized to PLA. This paper also estimated its catalyzing activity by integrating the performance index including the yield and quality(e.g., purity, crystal shape, etc) of lactide, depolymerization temperature of lactic acid oligomer, viscosity of the reaction system and the character of PLA synthesized by this lactide.The result shows that a more efficient processing way to synthesize lactide is: under reduced pressure, the lactic acid was dehydrated at 70-90C, for 1.5 hours; the dosage of catalyst is 0.8% of lactic acid while putting in evenly, then the temperature is risen slowly to 120~140C, to condensate for 3-4 hours; at last, the temperature is risen with the speed of 4C/10min in the range of 170-260C to depolymerizate. The crude yield of lactide reaches90%.The synthesis of polylactide by the ring-opening polymerization of lactide also involved in this present, and tried to do some researches of the separation of meso-Lactide and feasibility analysis, separating experiment and rectify theoretical calculation by FUC. When lactide with a content of about 63%, if in fact the temperature at 147C and at the pressure at 800Pa, the content of meso-lactide are attained 98% and 2% separately of tower top and tower kettle, the theoretical calculation shows that under minimum reflux ratio Rm=7, theoretical plate of complete reflux Nm=27, if practice reflux ratio R=10, theoretical number of plates N=73, adding material place lie in 31 plate from the tower bottom to top, material were entered by saturated gas. If this minimum number of plates can not be got with apparatus used, distillation may be carried out several times.
|
PDF Full Text Request