A new method has been invented for introducing texture, crimp, or set twist into synthetic polymer yarns which eliminates the need to heat the yarns prior to the texturing step. It has been found that the heat generated in the drawing step of synthetic yarn manufacturing processes is sufficient to cause a twisted undrawn yarn to retain the twist permanently after being drawn. The mild heat treatment experienced by the fibers is highly uniform and produces textured yarns of exceptional uniformity, especially with respect to dyeability, as compared with textured yarns produced by known art processes.

A process is described for false twist texturing polyester filament yarn at greatly enhanced speeds while inserting comparable twist levels and obtaining comparable yarn bulk without increasing yarn heat setting lengths. The process is particularly adopted to the utilization of partially oriented yarns in a draw texturing operation utilizing friction twisting as the means for simultaneously drawing and heat setting the yarn in a twisted state.

A false-twisted polyester yarn of good dyeing and mechanical properties is produced by a process wherein a yarn consisting of an as-spun polyester fiber having a mean birefrigence index (.DELTA.n) of from 15.times.10.sup.-3 to 150.times.10.sup.-3 is heat-treated and continuously, subjected to a false twisting treatment or draw-false twisting treatment. A preferred apparatus used comprises a feed yarn creel, a false twisting heater and an optional stabilizing heater, a false twisting element and a winder, wherein a heat-treating heater of the non-contact type is arranged upstream of the false twisting heater and between the false twisting heater and the feed yarn creel, at a height substantially equal to the height of the feed yarn creel so that the heat-treating heater confronts the false twisting heater; and a yarn cooling zone is provided between the heat-treating heater and the false twisting heater.

The invention relates to a process for making texturized multifilament yarns having profiled polyester filaments, by means of draw false twist texturizing, wherein feed yarns which are multifilament yarns having a high pre-orientation which corresponds to a birefringence of more than 65.times.10.sup.-3 are simultaneously drawn and false twist texturized without a reduction of the twist number but at temperatures which are no more than 30.degree. C. above the glass transition temperature (second order transition temperature) of the polymer and to the resulting luster yarns, the cross-section profiles of which are largely retained in this special texturizing process.

A multifilamentary polymeric yarn capable of undergoing false twist texturing is passed through a friction disc aggregate under conditions (as defined) which produce periodic slippage with respect to the friction disc aggregate whereby recurring relatively tightly twisted areas measuring approximately 0.125 to 1.25 inch are substantially maintained along the length of the yarn intermediate recurring textured areas measuring approximately 0.125 to 1.25 inch which include a twist in the opposite direction. The feed yarn may be either partially oriented yarn or a drawn yarn. In a preferred embodiment the feed yarn is a partially oriented polyethylene terephthalate multifilamentary yarn. Unmodified texturing machinery can be employed. The resulting yarn is space textured and may be utilized to form a crepe fabric having highly desirable aesthetic characteristics.