Biodegradable resin composition and molded body and production method thereof

The present invention relates to a biodegradable resin composition having excellent color tone, strength and gas barrier property and having improved durability under a moisture heat condition, a molded body made of the composition, and a production method of the composition.

In recent years, biodegradable resins typified by polylactic acid have attracted attention from the viewpoint of environmental conservation. Of the biodegradable resins, the polylactic acid is highly useful, because the polylactic acid has excellent transparency, is one of the most heat-resistant resins, is less costly due to the mass-producibility from vegetable-derived materials such as corn and sweet potato, and can contribute also to reduction of oil materials. However, when a molded body is produced by only the biodegradable resin, the molded body has insufficient long term storage stability and moisture heat resistance to cause problems such as the reduction of the strength and molecular weight caused by deterioration and the deteriorated appearance caused by the reduction of the molecular weight. Therefore, the molded body cannot endure applications where the molded body is repeatedly-used and prolonged use.

As a technique for solving this problem, JP-A-2001-261797 discloses that hydrolysis resistance is enhanced by blocking a carboxyl end of polylactic acid with a specific carbodiimide compound. As a method for enhancing the other performance of a biodegradable resin, JP-A-2001-49097 discloses that heat stability can be applied by adding a phenol phosphite compound to an aliphatic polyester resin.

As a method for enhancing the other performance of a biodegradable resin, JP-A-2002-338796 discloses that the strength and gas barrier property or the like of the biodegradable resin are enhanced by adding a layered silicate organically treated by specific ammonium ions to the biodegradable resin. JP-A-9-48908 discloses that hindered phenol, organic phosphite and a phosphonite compound are added to a thermoplastic polyester resin as a method for suppressing coloration when adding a layered silicate to the thermoplastic polyester resin. JP-A-2000-212422 discloses that the appearance and rigidity of a thermoplastic polyester resin made of diol and dicarboxylic acid can be enhanced by adding a layered silicate, one of benzophenone, benzotriazole and cyanoacrylate, and a phosphite compound to the thermoplastic polyester resin.

However, when the layered silicate is used for the polylactic acid resin, low-molecular weight substances and metal salts of lactic acid are deposited on the surface of the molded body during the long term storage of the molded body obtained using the resin to cause problems such as deteriorated appearance and reduced color tone caused by coloration by an ionic compound contained in the layered silicate. Unfortunately, the strength is reduced by the long term storage.

Even if the carbodiimide compound of JP-A-2001-261797 is added for preventing the strength reduction during the long term storage of the polylactic acid resin containing the layered silicate, the coloration of the polylactic acid resin is insufficiently suppressed in the presence of the layered silicate, and the effect for improving the strength reduction is also insufficient. Also, the generation of the deposited substance on the surface of the molded body during the prolonged storage cannot be suppressed.

Even if the phenol phosphite, the hindered phenol compound or the phosphonite compound and the like is applied to the polylactic acid as described in JP-A-2001-49097 and JP-A-9-48908, the problems such as the strength reduction and the deposited substances during the long term storage are not solved although the effect for improving the color tone is observed in some compounds.

Even if benzophenone, benzotriazole or cyanoacrylate, and the phosphite compound are added in the system containing the layered silicate as described in JP-A-2000-212422, the appearance is insufficiently improved, and the moisture heat resistance is unsuitable for practical use. In addition, JP-A-2000-212422 does not refer the polylactic acid which is a biodegradability polyester.

To solve the aforesaid problems, it is an object of the present invention to provide a biodegradable resin composition having excellent color tone, strength and gas barrier property and suppressing the strength reduction and the generation of deposited substances under a moisture heat condition.

As the results of the intensive studies for solving the problems, the present inventors have found that the initial color tone is improved by adding a carbodiimide compound and phosphite organic compound of a specified amount with a layered silicate to a biodegradable polyester resin consisting mainly of polylactic acid, and the strength reduction and the generation of deposited substances in high temperature and high humidity are suppressed, and attained the present invention.

The characteristic of the present invention is as follows.

(1) A biodegradable resin composition comprises:

100 parts by mass of a biodegradable polyester resin containing not less than 50% by mass of polylactic acid;

0.1 to 5 parts by mass of a carbodiimide compound;

0.1 to 10 parts by mass of a layered silicate; and

0.01 to 5 parts by mass of a phosphite organic compound.

(2) The biodegradable resin composition according to the item (1) comprises the phosphite organic compound and at least one additive selected from the group consisting of a hindered phenol compound, a benzotriazole compound, a triazine compound and a hindered amine compound in an amount of 0.01 to 5 parts by mass in total, instead of containing 0.01 to 5 parts by mass of the phosphite organic compound.

(3) The biodegradable resin composition according to the item (1) or (2), wherein the biodegradable resin composition has a strength-retaining rate of not less than 60% when being held at 60° C. and at a relative humidity of 95% for 300 hours.