Study On The Synthesis Process Of Degradable Plastic Poly Lactic Acid(PLA)

Since the beginning of 20th century, plastic has become a material which was used in almost every field. However, the biological system was greatly damaged for its undegradability. Poly(lactic acid)(PLA)polymers was made from renewable resources, and it can be degraded into water and carbon dioxide through the process of microorganism in nature. Because of its biodegradability and excellent mechanical properties, PLA is one of the most important alternatives to commodity petrochemical based plastics.Both direct synthesis and ring-opening polymerization (ROP) can be utilized to prepare homo- and copolymers of lactic acid (LA) such as L-lactide, D-lactide, D, L-lactide, and glycolide. The aim of the study is to develop the direct syntheses of poly lactic acid from an lactic acid oligomer in the absence of organic solvent via melt poly condensation. The viscosity-average molecular weights of polymers were determined. Their structure was confirmed by FTIR. It was found high molecular weight PLA could be got by melt polymerization in 10h, which distilled the lactic acid in vacuum system of pressure 640mmHg, temperature 60℃for 2h to 3h in advance, then polymerization was performed in nitrogen atmosphere, increase the temperature to 170℃, add the molecular sieve, keep stirring, meanwhile evacuated the system to 100Pa gradually.The molecular weight, microstructure, appearance, yields and degradability of polymers were investigated by using five kinds of catalyst(SnOct2,TSA,Zn,SnCl2,SnOct2+TSA). The results showed the best way to synthesize PLA(Mv10132) is using SnOct2toluene-p-sulfonic acid(0.4%Wt) as a compound catalyst.The hydrolytic degradation and thermal degradation of PDLLA were investigated. The hydrolytic degradation research results showed that the PDLLA molecular weight was first degraded, then the weight loss happened in normal saline, and the water solublecrystallizable oligomers were formed during the process of degradation. The thermal degradation research results showed that the thermal degradation of PDLLA is a random breaking process, the molecular weight was degraded at range of 100~250℃,only above 250℃,the weight loss was found.