The present invention relates to a spinning nozzle and more particularly to a spinning nozzle for controlling the cross-sectional structure of fiber obtained by melt-spinning a polymer which has a plate-like molecular structure.
As an example of the method for producing fiber by spinning an organic polymer, there has heretofore been used a method in which a molten organic polymer or a solution of the polymer in a suitable solvent is extruded through a die having a nozzle of a very small diameter.
In melt-spinning a polymer, especially a straight-chain polymer, the orientation in the direction of fiber axis has been considered to be an important factor which governs fiber characteristics. However, with regard to a polymer which has a plate-like molecular structure, it has become clear that with only control of the orientation in the direction of fiber axis, it is insufficient and that the orientation in the direction perpendicular to the fiber axis, namely in a cross-sectional direction of fiber, is also an important factor in determining fiber characteristics.
A conventional melt-spinning nozzle generally comprises a countersink 1, counterbore 2, an approach 3 and a capillary 4, as shown in FIG. 1. Its cross section is usually circular. There are also many conventional melt-spinning nozzles whose constituent parts cannot always be distinguished clearly, for example a countersink and a counterbore or an approach are integrated with each other.
However, even if a polymer having a plate-like molecular structure is melt-spun using such a conventional melt-spinning nozzle, it is very difficult to obtain a spun yarn of high performance. This will be explained below with respect to the case where a mesophase pitch is used as such polymer having a plate-like molecular structure.
If a mesophase pitch is melt-spun using a conventional melt-spinning nozzle, the resultant pitch fiber is apt to have a "radial structure" as shown in FIG. 2(a). Pitch fiber having such radial structure is apt to crack in the radial direction, and even if yarn cracking does not occur in the stage of pitch fiber, it will occur upon subsequent calcination (infusibilization or carbonization) into carbon fiber, thus making it impossible to develop a sufficient strength, and so its use as product becomes impossible. Therefore, at the stage of spinning a mesophase pitch which is apt to take such radial structure into pitch fiber, its cross-sectional structure should be controlled into such a "random structure" or "onion-like (or tangential) structure" as shown in FIG. 2(b) or 2(c) or into a structure which is basically a radial structure but is difficult to undergo yarn cracking. Whether such control can be made or not is very important in producing a high strength carbon fiber from mesophase pitch.
There are few examples of reports on this problem. In U.S. Pat. No. 4,376,747 there is disclosed a method in which a porous body of metal or ceramic material is formed into a portion of spinnaret in the interior of a die. An example of roughening the inner wall of a capillary is also disclosed therein. However, in performing on an industrial scale a multinozzle spinning in which a plurality of filaments are spun simultaneously from a single die, it is very difficult with the above method to have uniform the pressure drop in each nozzle, thus resulting in non-uniform volume of filaments extruded from the nozzles and so making it impossible to obtain a stable yarn of a uniform diameter.