CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2015-239688, filed on Dec. 8, 2015 and International Patent Application No. PCT/JP2016/086513, filed on Dec. 8, 2016, the entire content of each of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polylactic acid resin composition and a molded article formed from the composition. More specifically, the present invention relates to a polylactic acid resin composition capable of achieving both high crystallinity and transparency, and a molded article formed from the composition.

2. Description of the Related Art

Conventionally, a plastic has been used in fields of various casings, containers, and packages. Particularly, a transparent plastic material has been used for a container requiring capability of visually recognizing contents, such as a food container. As a transparent plastic material, polystyrene, polyethylene terephthalate, or the like has been used.

However, due to a recent change in a food culture or the like, a need for heat resistance of a plastic is increasing, for example, in a case where contents in a plastic container are warmed with a microwave oven. Polyethylene terephthalate having high transparency has heat resistance of about 60° C. or lower and cannot be heated with a microwave oven or the like. In addition, oriented polystyrene (OPS) has heat resistance of 80° C., and therefore oriented polystyrene is used for a top lid of a container heated with a microwave oven or the like. However, for example, in a case where the contents contain oil or heating time is long, the temperature may be easily higher than 80° C., and the OPS container may be damaged.

Meanwhile, in order to enhance heat resistance, it has been studied to orient polyethylene terephthalate (O-PET) or to make opaque polypropylene transparent with a transparent nucleating agent. However, each case has heat resistance of 110° C. or lower and a haze value indicating transparency of 10% or more, and there is no versatile plastic material that can achieve both transparency and heat resistance.

Meanwhile, examples of a highly transparent plastic material include a polylactic acid resin. The polylactic acid resin is a semi-crystalline material but is hardly crystallized. Therefore, the polylactic acid resin is amorphous in a normal molding cycle and has heat resistance of only about 50° C. Crystallinity can be enhanced by thermally treating (annealing) amorphous polylactic acid or molding amorphous polylactic acid with a high-temperature die of higher than 80° C. However, if crystallization is enhanced, amorphous polylactic acid usually becomes clouded, opalescent, and opaque. In addition, in crystallization in a die, molding and crystallization of a softened resin occur competitively. Therefore, there is a problem in moldability, for example, an edge of a molded article becomes more rounded than a designed shape to deteriorate shaping.

Patent document 1 describes a method for adding a fatty acid carboxylic acid amide to an aliphatic polyester such as polylactic acid. However, the degree of crystallinity reaches 33%, but a haze is 6.5%, and a result achieving both sufficient degree of crystallinity and transparency has not been obtained.

Patent document 2 describes a method for blending polyethylene glycol with a polylactic acid resin. However, patent document 2 describes improvement of liquidity, but does not describe improvement of crystallinity or maintenance of transparency.

Patent document 3 describes a method for adding a castor oil-based fatty acid ester to a polylactic acid resin. However, the degree of crystallinity of a molded article before a heat treatment is as low as less than 30%, and the haze value thereof is also more than 8% in all the cases. A result achieving both sufficient degree of crystallinity and transparency has not been obtained.

Patent document 4 describes a method for improving crystallinity and transparency by blending a plasticizer and a crystal nucleating agent containing a fatty acid amide with a biodegradable resin containing a polylactic acid resin. However, in this case as well, there is no example of achieving degree of crystallinity of more than 30% and a haze value of 5% or less, and a result achieving both sufficient degree of crystallinity and transparency has not been obtained.

CITATION LIST

Patent Document

[patent document 1] JP 9-278991 A

[patent document 2] JP 2012-158658 A

[patent document 3] JP 2006-143829 A

[patent document 4] JP 2007-262422 A

SUMMARY OF THE INVENTION

The present invention has been achieved in view of such circumstances, and an object of the present invention is to provide a polylactic acid resin composition or a molded article having good degree of crystallinity and excellent transparency.

In order to solve the above problems, in a method for obtaining a polylactic acid resin composition by melting a mixture obtained by mixing a specific plasticizer and lubricant with a poly-L-lactic acid resin, then cooling the mixture, and further heating the mixture, the present inventor has found that a transparent polylactic acid resin composition having high degree of crystallinity can be obtained by controlling all of the optical purity of L-lactic acid in a poly-L-lactic acid, the blending amounts of a plasticizer and a lubricant, a cooling temperature, and a heating temperature to specific conditions. By this method, a loose crystal system is constructed while transparency is maintained in the cooling process after melting, crystal is rearranged and recrystallized by further heating under the temperature conditions, and high degree of crystallinity that exhibits high heat resistance can be realized while transparency is maintained. The present invention has been completed based on these findings. The present invention is as follows.

That is, a polylactic acid resin composition according to a first aspect of the present invention is a polylactic acid resin composition obtained by melting a mixture containing 100 parts by weight of a poly-L-lactic acid (A) having an L-lactic acid purity of 99 mol % or more, 2.6 to 10 parts by weight of a plasticizer (B), and 0.3 to 1.0 part by weight of a lubricant (C), then cooling the mixture to 20 to 40° C., and then heating the mixture to 46 to 70° C., wherein the following conditions (a) and (b).

(a) A haze value at a thickness of 0.3 mm is 5% or less.

(b) The degree of crystallinity measured by a differential scanning calorimeter is 50 to 70%.

A polylactic acid resin composition according to a second aspect of the present invention is a polylactic acid resin composition obtained by melting a mixture containing 100 parts by weight of a poly-L-lactic acid (A) having an L-lactic acid purity of 99 mol % or more, 2.6 to 10 parts by weight of a plasticizer (B), and 0.3 to 1.5 parts by weight of a lubricant (C), then cooling the mixture to lower than 20° C., and then heating the mixture to 46 to 80° C., wherein the following conditions (c) and (d).

(c) A haze value at a thickness of 0.3 mm is 5% or less.

(d) The degree of crystallinity measured by a differential scanning calorimeter is 50 to 70%.

A polylactic acid resin composition according to a third aspect of the present invention is a polylactic acid resin composition obtained by melting a mixture containing 100 parts by weight of a poly-L-lactic acid (A) having an L-lactic acid purity of 98 to 99 mol %, 2.6 to 10 parts by weight of a plasticizer (B), and 0.3 to 1.0 part by weight of a lubricant (C), then cooling the mixture to 20 to 40° C., and then heating the mixture to 46 to 60° C., wherein the following conditions (e) and (f).

(e) A haze value at a thickness of 0.3 mm is 5% or less.

(f) The degree of crystallinity measured by a differential scanning calorimeter is 35 to 55%.

A polylactic acid resin composition according to a fourth aspect of the present invention is a polylactic acid resin composition obtained by melting a mixture containing 100 parts by weight of a poly-L-lactic acid (A) having an L-lactic acid purity of 98 to 99 mol %, 2.6 to 10 parts by weight of a plasticizer (B), and 0.3 to 1.5 parts by weight of a lubricant (C), then cooling the mixture to lower than 20° C., and then heating the mixture to 46 to 80° C., wherein the following conditions (g) and (h).

(g) A haze value at a thickness of 0.3 mm is 5% or less.

(h) The degree of crystallinity measured by a differential scanning calorimeter is 35 to 55%.

