Abstract: Undrawn preoriented polyester yarns containing 0.03 to 0.01% of fumed silica with a particle size of between 5 and 15 nm, exhibit a delay in crystallization.

Abstract: Process for improving the production efficiency of the spinning of an undrawn, preoriented yarn based on polyethylene terephthalate (PET) by the introduction, before the spinning, of 0.03 to 0.1% by weight of fumed silica with a particle size of between 5 and 15 nm into the molten polymer in the form of dispersion in a masterbatch of the same polymer as that to be processed, then spinning the PET containing the silica in the finely dispersed state, the filaments being next cooled by means of a gas stream at temperature between 17.degree. to 22.degree. C. and then sized and wound at a velocity of between 3500 and 5000 m/min.The gain in production efficiency, evaluated in relation to the shrinkage of the filaments in dry air at 180.degree. C., is at least 7%, generally at least 10-15%.