Since most plastics are light in weight, tough and durable and can be molded easily and arbitrarily, they are mass-produced to support our lives in many ways. However, when plastics are thrown away into the environment, they are not easily decomposed and are accumulated. When they are burnt, they emit a large amount of carbon dioxide, thereby accelerating global warming.
In view of this situation, researches into resins made from non-oil raw materials or biodegradable plastics which are degraded by microorganisms are now actively under way. The biodegradable plastics have an aliphatic carboxylate unit and are easily degraded by microorganisms. However, they have low heat stability and a serious problem such as a reduction in their molecular weights or the deterioration of their colors in the step where they are exposed to a high temperature, such as melt spinning, injection molding or melt film formation.
Although polylactic acid, out of the degradable plastics, is a plastic which has excellent heat resistance and good balance between color and mechanical strength, when it is compared with petroleum-based resins typified by polyethylene terephthalate and polyamide, there is still a world of difference in heat stability between it and the petroleum-based resins. To overcome this situation, various studies have been made on the improvement of the heat stability of polylactic acid. For example, Patent Document 1 proposes that a phosphoric acid-based compound or phosphorous acid-based compound is added to polylactic acid as a catalyst deactivator when the molecular weight reaches 50,000 or more. Patent Documents 2 and 3 teach that an acidic phosphate or chelating agent is added as a catalyst deactivator to improve the heat stability of polylactic acid. However, the addition of the catalyst deactivator to the low-molecular weight polylactic acid as in Patent Document 1 impedes a subsequent polymerization reaction, thereby making it impossible to obtain a high-molecular weight polymer. Meanwhile, the acidic phosphate disclosed by Patent Documents 2 and 3 causes the corrosion of production equipment or the deterioration of the hydrolytic resistance of the resin due to its acidity. The chelating agent enumerated above generally has low heat resistance and burns before a metal catalyst is supplied, thereby causing serious coloration and bad smell.
As described above, a polylactic acid composition which suppresses both opening polymerization caused by the residual polymerization catalyst and a reduction in molecular weight by the disconnection of the main chain and has excellent heat stability is not proposed yet.    (Patent Document 1) Japanese Patent No. 2862071    (Patent Document 2) Japanese Patent No. 3487388    (Patent Document 3) JP-A 10-36651