Conventionally, plastics are used as materials for various containers such as bottles and trays and various molded components in many fields. However, the plastics cause social problems associated with waste disposal and environmental pollution, because the plastics are used in a tremendous amount and most of the plastics are not biodegradable.
On the other hand, biodegradable plastics have recently been studied. Among the biodegradable plastics, biodegradable aliphatic polyesters have come into practical use. However, such a biodegradable aliphatic polyester generally has a lower melting point and a lower melt viscosity and, hence, is lower in heat resistance and mechanical strength. Further, the biodegradable aliphatic polyester has a lower crystallization speed, thereby presenting problems of drawdown and an insufficient expansion ratio in molding. For practical use, the biodegradable aliphatic polyester is required to have an improved melt tensile force, a strain hardening property in measurement of elongational viscosity, and an improved crystallization speed.
In general, addition of a polymer having a higher polymerization degree and use of a polymer having longer chain branches are considered to be effective for imparting the resin with the strain hardening property. However, preparation of the higher polymerization degree polymer requires longer time for polymerization, resulting in lower productivity. Further, the higher polymerization degree polymer is liable to be colored and decomposed due to a longer-period thermal history. Further, there is known a method for preparing polylactic acid in a branched aliphatic polyester by adding a multifunctional polymerization initiator (JP-A-10-7778 and JP-A-2000-136256). However, this method is problematic in that the introduction of branched chains in the polymerization makes it difficult to extract the polymer and to flexibly change the branching degree.
On the other hand, numerous studies are conducted on a method of crosslinking a common biodegradable resin by melt-kneading the biodegradable resin with a peroxide or a reactive compound, because this method is simple and allows for flexible adjustment of the branching degree. Use of an acid anhydride and a polyvalent carboxylic acid as disclosed in JP-A-11-60928 is not practical, because there is a variation in reactivity and a reduced pressure is required. Use of a polyvalent isocyanate as disclosed in JP-B-2571329 and JP-A-2000-17037 is an impractical and unestablished technique, because the molecular weight is liable to be reduced in re-melting and there is some problem associated with safety in handling. Gelation by crosslinking with the use of an organic peroxide or with the use of an organic peroxide and two or more compounds having unsaturated bonds as disclosed in JP-A-10-324766 is liable to suffer from polymerization marks, and makes it difficult to extract the polymer because of its higher viscosity. In addition, the productivity is lower, and the colorization and the decomposition are liable to occur. Further, the presence of the gel disadvantageously reduces the qualities of molded articles and foamed articles.
On the other hand, the biodegradable aliphatic polyester suffers from lower productivity in molding because of its lower crystallization speed. However, no drastic measures are taken to cope with this, but addition of inorganic particles is an only approach now under consideration for improving the crystallization speed.
In view of these problems, the inventors of the present invention propose a biodegradable polyester resin composition as a useful material, and a foamed article and a molded article produced from the composition in JP-A-2003-128901. However, the composition, the foamed article and the molded article each have a relatively high gel content. This makes it difficult for these articles to have satisfactory appearance and quality, limiting applications of the articles.