Cellulose ester fibers made from a cellulose acetate, cellulose acetate propionate, etc. have excellent features which synthetic fibers such as polyester fibers and polyamide fibers do not have. That is, cellulose ester fibers have many properties such as excellent gloss, profound color tone, color developability, dry feeling and moisture absorbability/releasability as clothing fibers, and therefore have been used particularly for high quality clothing.
In recent years, consumers' needs in the clothing field are oriented toward diversification and higher quality, and in this situation, studies are being made to use the features of both cellulose ester fibers and synthetic fibers such as polyethylene terephthalate fibers, for providing higher value-added fibers having the excellent features of both the types of fibers.
Such high value-added fibers proposed include novel cellulose ester fibers obtained by melt spinning (see Patent Documents 1 to 3). Patent Document 1 proposes cellulose acetate propionate fibers excellent in color developability and fast-drying, and Patent Documents 2 and 3 propose cellulose fatty acid mixed ester fibers with excellent mechanical properties. However, these cellulose ester fibers proposed have a problem that they cannot be dyed together with polyethylene terephthalate (PET) fibers, etc. in the same bath, since if they are dyed at a high temperature of 120° C., they decline in fiber strength so greatly that they cannot withstand practical use any more. Therefore, since it is difficult to mix the cellulose fatty acid mixed ester fibers and PET fibers or the like for obtaining a composite material for clothing, they are mostly used alone, being limited in widening applications. For reference, in the examples of Patent Document 1, the dyeing temperature of cellulose fatty acid mixed ester fibers (cellulose acetate propionate) was 90° C., and the dyeing temperature of PET (comparative example) was 130° C., indicating that it is difficult to dye both types of fibers in the same bath.
Further, another document (see Patent Document 4) proposes a molded article obtained by blending two cellulose esters different in substitution degree. However, this proposal does not suggest at all that a thermoplastic cellulose ester composition suitable for melt spinning can be obtained by adding a plasticizer satisfying a certain requirement and a cellulose acetate and that the obtained fibers are enhanced in mechanical properties after high temperature dyeing treatment.