Process for production of poly(lactic acid)-type resin, and poly(lactic acid)-type prepolymer

1. A method of producing a poly(lactic acid) resin comprising:

carrying out direct polycondensation using lactic acid as a main raw material with a tin organic carboxylate and methanesulfonic acid to prepare a crystallized prepolymer having a weight average molecular weight of 5,000 to 25,000, an enthalpy of fusion ΔHm of 50 to 65 J/g and an acid value A mol/ton satisfying (1):

450/(Mw/10,000−0.14)< A< 950/(Mw/10,000−0.14)  (1)

wherein Mw represents weight average molecular weight of said crystallized prepolymer and said crystallized prepolymer is a pellet having a maximum diameter of about 1.5 to 6 mm; and

subjecting said crystallized prepolymer to solid-phase polymerization.

2. The method according to claim 1 , wherein lactide content L of said crystallized prepolymer is 0.1 to 3.0 wt %.

3. The method according to claim 1 , wherein said crystallized prepolymer is poly-L-lactic acid or poly-D-lactic acid, and, in the poly-L-lactic acid, D-lactic acid content d is 0.2 to 2.0 mol %, and in the poly-D-lactic acid, L-lactic acid content 1 is 0.2 to 2.0 mol %.

4. The method according to claim 1 , wherein enthalpy of fusion ΔHm of said crystallized prepolymer is not less than 53 J/g.

5. The method according to claim 1 , wherein acid value A mol/ton of said crystallized prepolymer satisfies (2):

550/(Mw/10,000−0.14)< A< 850/(Mw/10,000−0.14)  (2).

6. The method according to claim 1 , wherein a temperature increasing rate in said solid-phase polymerization is not more than 10° C. per hour.

7. The method according to claim 1 , wherein final temperature in said solid-phase polymerization is 155 to 165° C.

8. A crystallized poly(lactic acid) prepolymer pellet containing a tin organic carboxylate and methanesulfonic acid having a maximum diameter of about 1.5 to 6 mm, a weight average molecular weight of 5,000 to 25,000, an enthalpy of fusion ΔHm of 50 to 65 J/g and an acid value A mol/ton satisfying (1):

450/(Mw/10,000−0.14)< A< 950/(Mw/10,000−0.14)  (1)

(wherein Mw represents weight average molecular weight of said crystallized prepolymer).

9. The crystallized poly(lactic acid) prepolymer pellet according to claim 8 , wherein said crystallized poly(lactic acid) prepolymer is prepared by carrying out direct polycondensation using lactic acid as a main raw material to obtain a prepolymer, and crystallizing said prepolymer.

10. The method according to claim 2 , wherein said crystallized prepolymer is poly-L-lactic acid or poly-D-lactic acid, and, in the poly-L-lactic acid, D-lactic acid content d is 0.2 to 2.0 mol %, and in the poly-D-lactic acid, L-lactic acid content 1 is 0.2 to 2.0 mol %.

11. The method according to claim 1 , wherein enthalpy of fusion ΔHm of said crystallized prepolymer is not less than 53 μg.

12. The method according to claim 3 , wherein enthalpy of fusion ΔHm of said crystallized prepolymer is not less than 53 J/g.

13. The method according to claim 2 , wherein acid value A mol/ton of said crystallized prepolymer satisfies the (2) below:

550/(Mw/10,000−0.14)< A< 850/(Mw/10,000−0.14)  (2).

14. The method according to claim 3 , wherein acid value A mol/ton of said crystallized prepolymer satisfies the (2) below:

550/(Mw/10,000−0.14)< A< 850/(Mw/10,000−0.14)  (2).