Studies On Preparing Biodegradable Copolyester Film

By conventional melt condensation polymerization the biodegradable aliphatic-aromatic copolyesters with high molecular weight can not easily be synthesized. The lower molecular weight leads these polymers not to be suitable for common processing operations, such as film blown, extrusion coating and blown molding, the applications of biodegradable copolyesters are therefore limited. On the other hand, the lower crystallization temperatures and slow crystallization speeds of these copolyesters will further worse their processabilities. In order to develop the biodegradable copolyester which can be used in flexible packaging, the processability of copolyester must be improved. The methods which could increases the molecular weight and regulate the crystallizing behavior of copolyester have been studied in this paper. It was found that the processability of biodegradable aliphatic-aromatic copolyester could be greatly improved by viscosity enhancement resulted from chain extension and grafting reaction in reactive extrusion, and the crystallization temperature could be obviously increased by the addition of nucleating agent.During the modification of copolyester the reactive extrusion in the presence of peroxide was the most effective method to increase molecular weight. It was shown that adding 0.05% liquid organic peroxides would decrease the melt index of copolyester from 47 g/10min. to 0.35 g/10min, however, in the case of using 1.2% chain extender, the melt index of copolyester was only just decreased to 19 g/min. The branching degree and gel fraction of copolyester modified by reactive extrusion were examined. The results had shown that the gel formation can be suppressed by careful selection of peroxide and by addition of a little amount of antioxidant. The long branching chains were formed on the main chain of copolyester during the reactive extrusion, these long branches will benefit the chain entanglement, thus increase melt strength and finally attribute to the bubble stability of film blown.It was found that ethylene-based copolymers can effectively nucleate the crystallization of copolyester. The experimental results have shown that the Surlyn resin produced by Du Pont has an excellent nucleator and a good compatibility with copolyester. When 5% of Surlyn, based on copolyester, was used as the nucleator, the crystallization temperature of copolyester could be increased up to 85-95°C, at which the freeze line of bubble would be more stable.During the film blown SiO₂ particles and erucamide were used as antiblocking agent and slip agent respectively. When these additives were loaded 1200 ppm and, 1200-2000 ppm in the copolyester, the produced film exhibited a good antiblock and slip properties, coefficient of friction of the film was between 0.4 and 0.2. According to the addition rate, antiblock and slip masterbatch with 10% active ingredient was prepared.The film blown operations of the reactive extrusion modified copolyester were conducted on the lab and industrial film blown equipment, the stable bubble shape were observed, the films have the excellent properties which are comparable to that of foreign commercial products.