It has been known that wholly aromatic polyamide fibers have various characteristics, such as excellent heat resistance and chemical resistance. Among them, a para-type wholly aromatic polyamide fibers are industrially used as a reinforcing material of various kinds of matrices and a rope owing to the excellent mechanical characteristics thereof, such as high strength and high elastic modulus.
However, in the case where the wholly aromatic polyamide fibers are heated in a high temperature atmosphere or used in a high temperature atmosphere, there is a problem that single fibers are fused to each other when the temperature is too high.
The wholly aromatic polyamide fibers are necessarily subjected to stretching at a high temperature and/or heat treatment for attaining high strength and high elastic modulus, but there is a problem that single fibers are fused to each other to fail to attain stable yarn-making or to reduce the mechanical characteristics of the resulting fibers. Furthermore, in the case where single fibers are partially fused to each other, the thread is lowered in flexibility to deteriorate the handleability thereof.
In order to solve the problems, JP-A-53-147811 and the like disclose a method of coating inorganic fine powder on wholly aromatic polyamide fibers having thermal fusibility before subjecting the fibers to heat stretching and/or heat treatment, whereby the fibers are prevented from being fused and simultaneously are improved in yarn-making property.
In the method, however, the inorganic fine powder coated on the fibers remain in a large amount after heat stretching and/or heat treatment, and thereby the method has a defect of causing such unfavorable effects in workability and adhesiveness that scums are liable to occur upon twisting the resulting fibers, and the adhesiveness to various matrices is liable to be lowered upon using as reinforcing fibers for rubber and resins.
In order to solve the problems, JP-A-62-149934 proposes a method of using particular inorganic fine powder, and after stretching and heat treatment, the inorganic fine powder coated on fibers is removed by subjecting the fibers to treatment of applying water and treatment of spraying an air stream. However, only by using the treatment of applying water and the treatment of spraying an air stream, it is difficult to remove the inorganic fine powder to a level capable of improving the workability sufficiently. The remaining amount can be decreased by repeating the treatments in plural times, but another problem occurs that the productivity is deteriorated to increase the cost.
Accordingly, it is the current situation that wholly aromatic polyamide fibers capable of providing such a high-performance product have not yet been proposed that is excellent in workability in various post-processes and is excellent in adhesiveness with various matrices.