Process of making conjugate fibers

1. A process for making a conjugate fiber, wherein at least one segment of the conjugate fiber is a PLA resin having, per gram of PLA resin in the segment, at least 20 J of crystallites having a melting temperature of at least 200° C., comprising

a) coextruding 1) a mixture of a high-D PLA starting resin and a high-L PLA starting resin with 2) a second resin which is not a mixture of a high-D PLA starting resin and a high-L PLA starting resin, to form a segmented extrudate in which at least one segment contains a mixture of a high-D PLA resin and a high-L PLA resin and at least one other segment contains the second resin;

b) cooling the extrudate to below the crystalline melting temperature of each of the PLA resins to form a conjugate fiber; and

c) heat treating at least the segment or segments of the conjugate fiber that contain the mixture of a high-D PLA resin and a high-L PLA resin at a temperature between the glass transition temperature of the PLA starting resins and the crystallization melting temperature of the PLA starting resins for a period of time such that the segments or segments containing the mixture of the high-D PLA starting resin and the high-L starting resin form, per gram of PLA resins in said segment or segments, at least 20 Joules of crystallites having a crystalline melting temperature of at least 200° C.

2. The process of claim 1 , wherein the mixture of the high-D PLA starting resin and the high-L PLA starting resin contains the high-D PLA starting resin and the high-L PLA starting resin at a weight ratio of from 40:60 to 60:40.

3. The process of claim 2 , further comprising d), after step b) or c), separating at least one segment containing the mixture of the high-D PLA resin and the high-L PLA resin from at least one segment containing the second resin.

4. The process of claim 3 wherein step d) is performed by dissolving at least one segment containing the second resin.

5. The process of claim 3 , wherein step d) is performed by melting at least one segment containing the second resin.

6. A process for making a microfiber of a polylactic acid stereocomplex, comprising

a) extruding an islands-in-the-sea type conjugate fiber, wherein the island portions of the conjugate fiber contain a mixture of a high-D PLA resin and a high-L PLA resin and the sea portion of the conjugate fiber contains a second resin and;

b) drawing the conjugate fiber such that the island portions of the conjugate fiber assume a thickness of 0.5 micron or less;

c) either prior to, during or after step b), heat treating the conjugate fiber at a temperature between the glass transition temperature of the PLA starting resins and the crystallization melting temperature of the PLA starting resins for a period of time such that the island portions of the conjugate fiber contain, per gram of PLA resin, at least 20 J of crystallites having a crystalline melting temperature of at least 200° C.; and following steps b) and c), and

d) separating the sea portion of the conjugate fiber from the island portions of the conjugate fiber to form microfibers corresponding to the island portions of the conjugate fiber.

7. The process of claim 6 , wherein the mixture of the high-D PLA starting resin and the high-L PLA starting resin contains the high-D PLA starting resin and the high-L PLA starting resin at a weight ratio of from 40:60 to 60:40.

8. The process of claim 6 , wherein in step b), the island portions of the conjugate fiber assume a thickness of from 10 to 300 nanometers.

9. The process of claim 6 , wherein in step b), the island portions of the conjugate fiber assume a thickness of from 20 to 100 nanometers.

10. A process for making a sheath-and-core conjugate fiber, comprising

a) extruding a conjugate fiber having a core portion and a sheath portion, wherein either the core or the sheath portion of the conjugate fiber contains a mixture of a high-D PLA starting resin and a high-L PLA starting resin and the other portion of the conjugate fiber contains a second resin;

b) drawing the conjugate fiber; and

c) either prior to, during or after step b), heat treating the portion of the conjugate fiber that contains the mixture of the high-D PLA starting resin and the high-L PLA starting resin at a temperature between the glass transition temperature of the PLA starting resins and the crystallization melting temperature of the PLA starting resins for a period of time such that such portion contains, per gram of PLA resin, at least 20 J of crystallites having a crystalline melting temperature of at least 200° C.

11. The process of claim 10 , wherein the mixture of the high-D PLA starting resin and the high-L PLA starting resin contains the high-D PLA starting resin and the high-L PLA starting resin at a weight ratio of from 40:60 to 60:40.

12. The process of claim 11 , wherein the sheath portion of the conjugate fiber contains a mixture of a high-D PLA resin and a high-L PLA resin.

13. The process of claim 12 , further comprising d) selectively removing the core from the conjugate fiber.

14. The process of claim 11 , wherein the core portion of the conjugate fiber contains a mixture of a high-D PLA resin and a high-L PLA resin.