1.  Field of the Invention
The present invention relates to acrylic synthetic fibers having an irregular deeply uneven section the surface of which is covered with discontinuous crease parts to form deep unevenness, which are quite different from acrylic fibers having a round section or a dog bone section developed hitherto, and, particularly, to acrylic synthetic fibers suitable for spring clothes and summer clothes which have a ramie-like feeling and a linen-like luster. 
In acrylic synthetic fibers, it is very difficult to produce fibers having an irregular-form section by a spinning operation because of characteristics of the polymer, and the fibers generally have a round section or a dog bone section.  Further, the acrylic fibers have not only the above described shape of the section but also a peculiar warm feeling themselves.  Accordingly, they have an aptitude as winter clothes but do not have an aptitude as summer clothes.  Therefore, the acrylic fibers are subject to restriction in the viewpoint of their use, and it has been greatly desired to develop the use of acrylic fibers so as to obtain acrylic fibers having an aptitude as summer clothes.  However, acrylic fibers having such characteristics have not been developed yet. 
It is an object of the present invention to develop acrylic fibers suitable for raw materials of summer clothes which have a ramie-like feeling characterized by a dry and fresh feeling and a peculiar elegant linen-like luster by which the use of acrylic fibers is expanded. 
A characteristic of the present invention is the fibers which have an irregular form section wherein at least two convex-concave parts having an unevenness value represented by l/D.sub.1 of 0.05 or more are present in the section of fibers and the surface of fibers is covered with many creases having an average width of 1.mu.  or more which are provided along the lengthwise direction of the fibers. 
Another characteristic of the present invention is a process for producing the above described fibers wherein a wet spinning process is utilized, a spinning nozzle having a triangular orifice is used as the spinning nozzle, a spinning solution dope having a low viscosity, particularly, that having a viscosity of 30 to 120 poises which is not used for the prior acrylic fiber producing process by a wet spinning is used as the organic spinning solution dope, and drying of fibers after conclusion of spinning is carried out at 105.degree.  to 160.degree.  C., by which fibers having the above described specified irregular form section are produced. 
A further characteristic of the present invention is to blend yarns having an excellent water absorption property and good physical properties which comprise 50 to 90% of substantive fibers having the above described specified irregular form section which does not contain voids and 10 to 50% of fibers having the above described specified irregular form section which contains capillary voids. 
2.  Description of the Prior Art
Acrylic fibers developed hitherto have been advantageously utilized as raw materials for winter clothes, because the most typical characteristic of the fibers is to have a warm wool-like feeling.  However, since acrylic fibers are wool-like fibers having a warm touch or a slipperious feeling and they have characteristics such as an inferior moisture absorbing or diffusing property, etc.,  they are not suitable as raw materials for spring clothes and summer clothes.  Therefore, it has been believed that application of acrylic fibers is inferior to that of polyester fibers. 
Various proposals for giving a sweat absorbing property to the acrylic fibers have been done, one of which is seed-core type acrylonitrile fibers described in Japanese Patent Application (OPI) No.  149923/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").  In that invention, the core component of the fibers is composed of a copolymer of acrylonitrile and a comonomer represented by the formula ##STR1## and the core component has a void structure.  Although these acrylonitrile fibers have a good hygroscopic property as compared with acrylonitrile fibers developed hitherto, the state of the surface thereof is similar to that of the prior fibers and the fibers are still wool-like. 
Further, U.S. Pat.  No.  3,929,946 discloses hygroscopic acrylic fibers containing 63% or less of capillary voids, wherein the free surface area including the surface of voids is 10.times.10.sup.4 to 1.7.times.10.sup.6 cm.sup.2 /g.  These fibers have a good characteristic in the viewpoint of hygroscopic property, but they are not suitable as raw materials of summer clothes because of having a wool-like feeling. 
Furthermore, it has been proposed in British Pat.  No.  1,532,770 a process for producing fibers having a core-jacket structure and containing microvoids in the core part which comprises carrying out dry spinning of a spinning solution dope composed of an acrylonitrile polymer and a solvent to which polyhydric alcohol, methyl hydroxybenzoate, isophthalic acid, pyromellitic acid, zinc chloride or magnesium chloride, etc.,  is added.  The fibers obtained by this process has a good hygroscopic property, but the hand and the external appearance of them are no better than wool-like feelings. 
Further, it has been studied to develop acrylonitrile fibers having an uneven surface. 
One of such studies has been described in Japanese Patent Application (OPI) No.  158322/80, which discloses a process for producing acrylonitrile fibers having an aptitude of artificial hairs which comprises spinning a spinning solution dope obtained by dissolving an acrylonitrile polymer composed of 30 to 55% by weight of acrylonitrile and 45 to 70% by weight of vinylidene chloride or vinyl chloride in an acetone solution by a wet spinning process.  Fibers obtained by this process have an uneven pattern on the surface thereof and have good properties for artificial hairs, but they do not have fresh and dry feelings but have wool-like feelings.  The polymer constituting these fibers are not suitable as fibers for cloths, because an amount of acrylonitrile copolymerized is as low as 55% by weight.  When it is intended to produce fibers of an acrylonitrile polymer having a higher acrylonitrile content, a spinning solution dope cannot be formed because the acrylonitrile polymer does not dissolve in acetone, and, consequently, the objective fibers cannot be produced. 
Moreover, Japanese Patent Application (OPI) No.  15832/80 discloses acrylonitrile fibers having two or more macrovoids having a longer diameter of 2.mu.  or more in the section of the fibers the surface of which has numbers of creases having an average width of 0.1 to 5.mu.  provided along the lengthwise direction.  These fibers are useful because of having cotton-like feelings, but a linen-like feeling and a ramie-like luster are not obtained yet because uneven creases formed on the surface are not deep.  Accordingly, they are not sufficient as raw materials for summer clothes.  Further, fibers containing a large volume of voids in the interior are not sufficient in the viewpoint of physical fibrous properties.