Method for producing crystallized polyester

1. A method for producing a crystallized polyester, which comprises a crystallization step of applying a shear and/or a pressure to a polyester, wherein the polyester is polylactic acid, in a molten state at a temperature of (Tm−70° C.) to (Tm−5° C.), where Tm is a melting point of the polyester, thereby converting the polyester into a state having a crystallinity of 10% or more and fluidity, wherein the state of having fluidity is a state having a melt viscosity of 1000 Pa·s or less, and the polyester to be subjected to the crystallization step has a weight average molecular weight of 5,000 to 100,000.

2. The method for producing a crystallized polyester according to claim 1 , wherein the polyester to be subjected to the crystallization step has a crystallinity of less than 10%.

3. The method for producing a crystallized polyester according to claim 1 , wherein the shear of a shear rate of 10 to 400/second and/or the pressure of 0.05 to 10 MPa is applied.

4. The method for producing a crystallized polyester according to claim 1 , wherein the time of applying the shear and/or the pressure is from 0.1 to 10 minutes.

5. The method for producing a crystallized polyester according to claim 1 , further comprising the step in which the polyester to be subjected to the crystallization step is poly-L-lactic acid or poly-D-lactic acid and, when the polyester is poly-L-lactic acid after converting the polyester into a state having a crystallinity of 10% or more and fluidity by the crystallization step, poly-D-lactic acid is mixed and, when the polyester is poly-D-lactic acid, poly-L-lactic acid is mixed.

6. A method for producing a crystallized polyester, which further comprises a step of mixing the crystallized polyester obtained by the method according to claim 1 with another resin.

7. The method for producing a crystallized polyester according to claim 1 , which further comprises steps of cooling and solidifying, and then pelletizing the crystallized polyester after the crystallization step.

8. A method for producing a crystallized polyester, which further comprises a step of subjecting the crystallized polyester obtained by the method according to claim 1 to solid phase polymerization.

9. A method for producing a crystallized polyester, which further comprises a step of subjecting the crystallized polyester according to claim 5 to solid phase polymerization, and in the step of mixing poly-L-lactic acid and poly-D-lactic acid, at least one of poly-L-lactic acid and poly-D-lactic acid satisfies the following equation (1):

(Δ Hm−ΔHc )>20 (J/g)  (1)

where

ΔHm: crystal melting enthalpy (J/g), and

ΔHc: crystallization enthalpy (J/g) upon temperature increase.

10. The method for producing a crystallized polyester according to claim 1 , wherein polyester to be subjected to the crystallization step is an oligomer or a prepolymer of polylactic acid.

11. A method for producing a crystallized polyester according to claim 1 , which comprises a polymerization step of reacting a lactic acid to produce an oligomer or a prepolymer; a crystallization step of applying a shear and/or a pressure at a temperature of (Tm−70° C.) to (Tm+20° C.), where Tm is a melting point of the oligomer or prepolymer, thereby converting the polyester into a state having a crystallinity of 10% or more and fluidity; a step of cooling and solidifying, and pelletizing the obtained crystallized polyester; and a step of subjecting the obtained pellet to solid phase polymerization; in this order, wherein the oligomer or prepolymer obtained by the polymerization step is subjected to the crystallization step without solidifying.

12. The method for producing a crystallized polyester according to claim 11 , wherein the crystallization step is carried out using a melting machine, and a temperature of the melting machine in the crystallization step is controlled according to a pressure of a resin measured by a pressure gauge mounted to at least one of the melting machine and pelletizers used in the said pelletize step, wherein the temperature of the melting machine is lowered when the pressure decreases and the temperature of the melting machine is raised when the pressure increases.