Elastic knitting fabric having multilayer structure

1. An elastic knitted fabric comprising a multilayer structure which is made by binding together separate front and back ground knitted fabrics each of which is formed by one needle bed of a knitting machine having two needle beds, wherein at least one of the ground knitted fabrics form stitches in a state in which polyurethane based elastic fibers as a bare string and a non-elastic yarn are arranged and said ground knitted fabrics are bound with only a binding yarn(s) constituted by a polyurethane based elastic fiber bare string(s) of 17 to 3000 decitexes.

2. The elastic knitted fabric according to claim 1 , wherein said elastic knitted fabric is a circular knitted fabric with the separate front and back ground knitted fabrics each formed by one needle bed, the ground knitted fabrics being bound together by a tuck loop of only a binding yarn(s) constituted by a polyurethane based elastic fiber bare string(s) of 33 to 3000 decitexes, wherein either of said separate front and back ground knitted fabrics has a smaller loop length than the other, the binding yarn(s) is bound to 25% or more of stitches of one of the separate front and back ground knitted fabrics having a lower stitch density, a ratio of the loop length of the binding yarn(s) to the loop length of the one of said separate front and back ground knitted fabrics having a smaller loop length is in a range of 0.6 to 2.3, and the elastic knitted fabric has an air gap between the front and back ground knitted fabrics and has a three dimensional structure.

3. The elastic knitted fabric according to claim 1 , wherein said elastic knitted fabric is a circular knitted fabric with the separate front and back ground knitted fabrics each formed by one needle bed, the ground knitted fabrics being bound together with only a binding yarn(s) constituted by a polyurethane based elastic fiber bare string(s) of 17 to 1500 decitexes, wherein either of said separate front and back ground knitted fabrics has a smaller loop length than the other, at least one of the ground knitted fabrics is bound by a tuck loop to the binding yarn(s), the binding yarn(s) is bound to 25% or more of stitches of one of the separate front and back ground knitted fabrics having a lower stitch density, a ratio of the loop length of the binding yarn(s) to the loop length of the one of said separate front and back ground knitted fabrics having a smaller loop length Is in a range of 0.2 to 0.6, and the elastic knitted fabric has a three-layer structure.

4. The elastic knitted fabric according to claim 1 , wherein each of said ground knitted fabrics has a warp-knitted structure, either of said separate front and back ground knitted fabrics has a nailer loop length than the other, and said binding yarn(s) is bound to 25% or more of stitches of one of the separate front and back ground knitted fabrics having a lower stitch density.

5. The elastic knitted fabric according to any one of claims 1 to 4 , wherein at least one of the ground knitted fabrics contains an elastic composite yarn.

6. The elastic knitted fabric according to any one of claims 1 to 4 , wherein both the front and back ground knitted fabrics form stitches in a state in which polyurethane based elastic fibers as a bare string and a non-elastic yarn are arranged.

7. The elastic knitted fabric according to one of claims 1 to 4 , wherein the size of the polyurethane based elastic fibers in the at least one ground knitted fabric is (D-g), and the size of the polyurethane based elastic fibers in the bare string(s) of the binding yarn(s) is (D-c), and the ratio (D-c/D-g)≧2 is satisfied.

8. The elastic knitted fabric according to any one of claims 1 , 2 , or 4 , wherein the ratio of the gross size of a binding yarn(s) for binding 1 cm 2 of ground knitted fabric: D (decitex) to the thickness of the elastic knitted fabric: T (mm) is:

5×10 3 ≦D/T≦5×10 5 .

9. The elastic knitted fabric according to claim 6 , wherein the elongations in warp and weft directions are each 80 to 150% under a load of 3.5 N/cm and 100 to 200% under a load of 9.8 N/cm, an elongation ratio(A) and an elongation ratio (B) in warp and weft directions, expressed by the following equations (1) and (2) are each in a range of 0.6 to 1.2:

elongation ratio ( A )=elongation (%) in warp direction under load of 3.5 N/cm÷elongation in weft direction (%);  (1)

elongation ratio ( B )=elongation (%) in warp direction under load of 9.8 N/cm÷elongation in weft direction (%).

10. The elastic knitted fabric according to any one of claims 1 to 4 wherein the elastic knitted fabric has a recessed portion or raised portion formed and fixed by heat molding, and has a three-dimensional structure.

11. The elastic knitted fabric according to claim 10 , wherein the volume retaining factor of a mold molded by heat molding, defined below, is 0.5 or greater:

volume retaining factor=(molding volume retained by molded elastic knitted fabric)/heat molding volume).

12. The elastic knitted fabric according to any one of claims 1 to 4 , wherein two or more types of non-elastic yarns are used in at least one of the ground knitted fabrics, and a jacquard pattern is provided by the two or more types of non-elastic yarns.

13. The elastic knitted fabric according to claim 2 or 4 , wherein a part of the elastic knitted fabric having a three-dimensional structure and an air gap between the front and back two ground knitted fabrics is bound in a contact state with a non-elastic yarn forming a part of the binding yarn(s) and/or a ground knitted fabric.

14. A molded cloth, at least part of which is formed by a non-sewn cylindrical circular knitted fabric, wherein the cylindrical circular knitted fabric is the elastic knitted fabric according to any one of claims 1 to 4 .

15. A process for manufacturing the elastic knitted fabric according to any one of claims 1 to 4 comprising knitting a knitted fabric containing an elastic yarn by a circular knitting machine, wherein a feed speed (V-g) of a bare string of polyurethane based elastic fibers for knitting a ground knitted fabric is unequal to a feed speed (V-c) of a bare string of polyurethane based elastic fibers of the binding yarn(s) for binding the front and back ground knitted fabrics.

16. The process according to claim 15 , wherein the bare string of polyurethane based elastic fibers of the binding yarn(s) for binding the front and back two-layer ground knitted fabrics is fed at a controlled draw ratio of 2 or less.