Preparation And Properties Of Polylactide/Nano-silica In Situ Composites

In this thesis, in situ melt polymerization of D, L-lactide (DLLA) and modified nano-silica in the presence of catalyst has been proposed as a novel approach to prepare PLA/nano-silica composites. The preparation process was simplified and the properties were improved.Firstly, D, L-lactide was prepared from D, L-lactic acid with zinc oxide as catalyst. Effects of dehydration time of lactic acid and pyrolytic temperature on crude yield of DLLA were studied. The results showed that the optimal conditions were as follows: liquid temperature was heated to 145.0℃under atmospheric dehydrated condition, then added zinc oxide amount of which was 1.5wt%, vacuum dehydration temperature was 150.0℃for 2 hours, the range of depolymerizing temperature was 170℃~250℃,crude yield of DLLA was 85%. Ethanol was used as solvent for recrystallization of DLLA. Time for recrystallization was shortened because of using heating jacket. The total yield of purified lactide increased to 70% after three recrystallizations.Then, nano-silica was modified by coupling agentγ-glycidoxypropyl trimethoxysilane (KH-560). Influences of mass ratio of nano-SiO2/KH-560 were investigated. The optimal content of KH-560 was 10wt%.Preparation methods of PLA/nano-silica were discussed. The influences of catalyst species and quantity, reaction temperature and time, content of nano-silica on the weight average molecular weight ( M w) of composites were discussed. The resules of orthogonal tests showed that the optimal reaction conditions were as follows:tin octoate was used as catalyst; W(SnOct2)/W(DLLA) and W(nano-SiO2)/W(DLLA) was 1:400 and 1:20, respectively; the reaction temperature was 140℃and the reaction time was 72h. Under this conditions, composites with M w of 1.5091×105 was obtained. The results of FTIR and SEM showed that nano-silica and polylactide were connected with covalent bonds, and inorganic phases were dispersed in polylactide homogeneously. The results of TG indicated that thermal decomposition temperature was increased with the increasing of silica content. The tensile strength of composites was enhanced because of adding nano-inorganic particles into polylactide. The water absorption results showed that water absorption property of PLA was improved by adding modified nano-silica which could accelerate degradation of PLA and enhance compatibility. The results of ultraviolet spectro-photometric analysis of composites indicated that light transmittance of composites was similar to that of pure PLA. The existence of nano-silica had non clear influence on the light transmittance of composites.