Abstract: The invention pertains to a process for determining the dye uptake of polyethylene terephthalate fibers in which the fibers are irradiated with high-intensity monochromatic light. A light-sensitive sensor is used to measure the Raman scattering and compare it with that of an earlier established model on the basis of Raman spectra of polyethylene terephthalate fibers of known composition and structure. A model is used to calculate the density, which constitutes a quantitative measure of the dye uptake. In the calculation use is made of measurement points from a number of regions in the spectral measuring region of 598 to 1900 cm−1.

Abstract: Multifilament yarn, monofilament, or film, formed at least substantially of polyethylene naphthalate, has a strength of at least 400 MN/tex and a DSC melting point of at least 292.degree. C., where the DSC melting point is measured on a sample quantity of 2.5 mg at a heating rate of 20.degree. C. per min. For production, melted polyethylene naphthalate is extruded from a multi-hole spinneret, the filaments or film, solidified by cooling, are/is withdrawn at a rate of 500 to 10,000 m/min, where the filaments or film are/is subjected to a draw-down B of 40 to 25,000 from the spinneret output to the first withdrawal element. The polymer used may contain at least 85% by weight polyethylene naphthalate and have a melt flow index of 3 to 26.

Abstract: Multifilament yarn, monofilament, or film, formed at least substantially of polyethylene naphthalate, has a strength of at least 400 mN/tex and a DSC melting point of at least 292.degree. C., where the DSC melting point is measured on a sample quantity of 2.5 mg at a heating rate of 20.degree. C. per min. For production, melted polyethylene naphthalate is extruded from a multi-hole spinneret, the filaments or film, solidified by cooling, are/is withdrawn at a rate of 500 to 10,000 m/min, where the filaments or film are/is subjected to a draw-down B of 40 to 25,000 from the spinneret output to the first withdrawal element. The polymer used may contain at least 85% by weight polyethylene naphthalate and have a melt flow index of 3 to 26.