Composite fibers

1. A composite fiber comprising a polyester component and a polyamide component bound in a side-by-side or eccentric core-in-sheath structure,

in which

the polyester component is a modified polyester having an intrinsic viscosity (IV) of 0.30-0.43 and comprising 5-sodiumsulfoisophthalic acid copolymerized in an amount of 2.0-4.5 molar % based on the acid component, and

which composite fiber exhibits a percentage of crimp DC of 1.3-15.0% after the composite fiber is treated in boiling water for 30 minutes under a load of 1.76×10 −3 cN/dtex, and then dry heat treated for 30 minutes at 100° C. under a load of 1.76×10 −3 cN/dtex for stabilization of the crimps and further dry heat treated for one minute at 160° C. under a load of 1.76×10 −3 cN/dtex, a percentage of crimp HC of 0.5-10% after the crimped composite fiber is immersed in water at 20-30° C. for 10 hours, and a difference ΔC between the percentages of crimp DC and HC of 0.5-7.0%, as defined represented by the following equation:

Δ C (%)= DC (%)− HC (%).

2. A composite fiber according to claim 1 , wherein the tensile stress under 10% elongation of the composite fiber is 1.6-3.5 cN/dtex.

3. A composite fiber according to claim 1 , wherein the tensile strength is a tensile strength of 3.0-4.7 cN/dtex.

4. A combined filament yarn comprising a composite fiber according to claim 1 and a different type of fiber with a smaller boiling water shrinkage.

5. A combined filament yarn comprising a composite fiber according to claim 1 and a different type of fiber with a larger boiling water shrinkage.

6. A false twisted yarn obtained by supplying a composite fibers comprising a polyester component and a polyamide component bound in a side-by-side or eccentric core-in-sheath structure to a false twisting step,

(1) wherein the polyester component is a modified polyester having an intrinsic viscosity (IV) of 0.30-0.43 and comprising 5-sodiumsulfoisophthalic acid copolymerized in an amount of 2.0-4.5 molar % based on the acid component, and

(2) the composite fibers in the false twisted yarn exhibit, after the false twisted yarn is treated in boiling water for 30 minutes under a load of 1.76×10 −3 cN/dtex, and then dry heat treated for 30 minutes at 100° C. under a load of 1.76×10 −3 cN/dtex for stabilization of the crimps and further dry heat treated for one minute at 160° C. under a load of 1.76×10 −3 cN/dtex, a percentage of crimp TDC of 10-30%, the fibers in the resultant crimped false twisted yarn exhibit, after the crimped false twisted yarn is immersed in water at 20-30° C. for 10 hours, a percentage of crimp THC of 5-17%, and the difference in percentage of crimp ΔTC represented by (TDC(%)−THC(%)) is 3-15%.

7. A false twisted yarn according to claim 6 , wherein the composite fiber supplied to the false twisting step exhibits, after the composite fibers are treated in boiling water for 30 minutes under a load of 1.76×10 −3 cN/dtex, and then dry heat treated for 30 minutes at 100° C. under a load of 1.76×10 −3 cN/dtex for stabilization of the crimps and further dry heat treated for one minute at 160° C. under a load of 1.76×10 −3 cN/dtex, a percentage of crimp DC of 1.3-15.0%, and, after the crimped composite fibers are immersed in water at 20-30° C. for 10hours, a percentage of crimp HC of 0.5-10%, and a difference ΔC between the percentage of crimps DC and HC of 0.5-7.0%.

8. A composite fiber according to claim 1 , wherein the tensile stress under 10% elongation of the composite fiber is 1.6-3.5 cN/dtex.

9. A composite fiber according to claim 1 , wherein the tensile strength is a tensile strength of 3.0-4.7 cN/dtex.

10. A composite fiber according to claim 2 , wherein the tensile strength is a tensile strength of 3.0-4.7 cN/dtex.