In recent years, the mass consumption-induced exhaustion of fossil resources typified by petroleum and the global warming caused by an increase in the carbon dioxide concentration have arisen as problems. This has led to a worldwide attempt to replace the petroleum-derived resins for general-purpose use with plant resins such as polylactic acids, which are derived from plants. Polylactic acids are made from plants such as corn, which do not worry us about exhaustion thereof, and after being disposed, they are decomposed into water and carbon dioxide, which are nontoxic, by workings of microorganisms in soil. Further, polylactic acids are circulating-type materials such that water and carbon dioxide generated by incineration thereof are returned into plants again by photosynthesis, and hence, they place only low load on the environment.
Recently, it is proposed to use plant resins principally containing polylactic acids even for forming enclosures for electronic equipment such as notebook personal computers, cellular telephones, etc. (see Patent Document 1).
Though possessing high stiffness such as flexural strength, polylactic acids as resins however have low impact resistances such as Izod impact strength, and low heat resistance such as load deflection temperature. Therefore, it is difficult to use it alone for forming enclosures for electronic equipment. As one of solutions of this problem, a polymer alloy is proposed, which is obtained by replacing a part of a polylactic acid with a petroleum-derived thermoplastic resin that has excellent impact resistance and heat resistance (see Patent Documents 2, 3, and 4).                Patent Document 1: JP 2001-244645 A        Patent Document 2: JP 2003-138119 A        Patent Document 3: JP 2003-342452 A        Patent Document 4: JP 2004-277497 A        
The Patent Document 2 describes resin compositions in which a polylactic acid, a polyacetal resin, and an acrylic resin are used as its ingredients, thus proposing resin compositions that have excellent moldability, processability, mechanical characteristics, and heat resistance.
The Patent Document 3 describes resin compositions containing a polylactic acid-based resin, an aliphatic polyester other than polylactic acid-based resins, and a plasticizer, thus proposing resin compositions that have excellent impact resistance and heat resistance.
The Patent Document 4 describes resin compositions containing a polylactic acid-based resin, an aliphatic polyester other than polylactic acid-based resins, and an inorganic filler, thus proposing resin compositions obtained by improving the impact resistance and heat resistance of the polylactic acid-based resin.
A polymer alloy of a polylactic acid and a thermoplastic resin, as described above, generally is obtained through a kneading step in which a pellet of a polylactic acid and a pellet of a thermoplastic resin are molten at a high temperature and mixed. In the kneading step, if a polylactic acid and a thermoplastic resin are not mixed finely and homogeneously, the properties such as impact resistance and heat resistance of the polymer alloy thus obtained cannot be enhanced. Conventionally, there are many obstacles to the mixing of a polylactic acid and a thermoplastic resin, because of their polarity or non-polarity, and it is difficult to form an ideal polymer alloy. Therefore, an impact resistance and a heat resistance obtained with a polymer alloy of a polylactic acid and a thermoplastic resin cannot be as high as expected.