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Blends of polyoxymethylene (POM) with a copolymer of ethylene and vinyl acetate (CEVAc) have been studied. The effect of viscosity ratio for melts of the components on the processes of fiber formation in extrudates and on the rheological properties of the molten blend has been tested. The viscosity ratio of the fiber‐forming POM and the matrix varied in the range 0.35‐27.7. POM ultrathin fibers of unlimited length can be formed in the CEVAc only at a viscosity ratio close to unity. For ratios much greater than unity, the extrudate is found to contain short fibers and a finely dispersed powder or no fibers at all. If the viscosity of the POM melt is lower than that of the matrix, films are formed in addition to fibers. The second factor that governs fiber formation is the extrusion shear stress. An optimum shear stress exists at which the amount of ultrathin fibers is a maximum.

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The fiber‐forming properties of the following polymer mixture melts have been investigated: polypropylene‐copolyamide (PP/CPA), polyoxymethylene‐copolyamide (POM/CPA), POM‐copolymer of ethylene and vinyl acetate (POM/EVAC), PP/EVAC, EVAC/CPA, and polypropylene‐polyvinylalcohol (PP/PVA). The capability of the polymer mixture melts to fiber‐form was estimated by degree of the polymer melt longitudinal deformation. It has been determined that the fiber‐forming properties of polymer mixture melts can be regulated by changing the chemical nature of the mixture components, structure‐formation processes and by introducing various additives of polymer and non‐polymer nature into binary mixture. The mechanism of action of additives is connected with changing microrheology processes during mixture melt flowing under the influence of the specific interaction between polymer macromolecules and additives at the interphase. The complex threads from PP, POM or EVAC microfibers have been produced and the features of their structure and properties have been investigated. There are the following distinctive properties of the complex threads consisting of microfibers: high strength and initial modulus, high elasticity, exclusive softness, pleasant feel, coupling, capaciousness, and wool‐likeness without special textural procedures, high sorption of moisture, dyes and other substances.