Supercritical CO fluid, a new environmentally friendly dyeing medium, changes the fiber structure to a certain extent in dependence on the treatment temperature and pressure used. Therefore the changes of crystalline structure in poly(ethylene terephthalate) (PET) fibers as brought about under the influence of supercritical CO fluid were investigated. For the data collection of wideangle X-ray diffraction full patterns a two-circle goniometer, equipped with a position sensitive detector, was used. From the observed twodimensional fiber diffraction patterns the crystallinities of various treated fibers were evaluated. The equatorial scanning yielded the dimensions of crystallites. To elucidate the fibersurface morphology changes SEM analyses were performed.The supercritical fluid dyeing of PET fibers with highly developed microfibrillar structure under tautends conditions promotes changes which are characterized by an increase in crystallinity and by diminution of the apparent crystallite dimensions. Some changes of surface morphology of dyed fibers were observed as well.

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The application of supercritical fluids (SCF) to the technological processes of fibre treatment is becoming increasingly important. Therefore we investigated highly oriented semicrystalline unmodified poly(ethylene terephthalate) fibres by means of SAXS in order to study the influence of supercritical CO2 (critical data: Tc = 304.2 K, Pc --7.38 MPa), an environmentfriendly dyeing media, on the lamellar structure and on the content, size and shape of microvoids in the fibres.Blind dyeing was performed in an autoclave after two different methods: the isothermal-isobaric method and the temperature-pressure step method.To elucidate the effects of the treatment conditions on the microvoid system and the lamellar colloid structure, scattering curves of untreated and treated fibres were determined in equatorial and meridional directions, and small-angle x-ray photographs were taken as well.Several parameters pertaining to the void system and the long period of the fibres were derived therefrom. The results suggest that the microvoid system of the fibres is more influenced by the SCF than the lamellar fibre structure itself. As the treatment conditions in SCF were adapted to the dyeing conditions, they are probably too low to cause considerable changes in the two-phase fibrillar structure. Within the treatment levels used only the highest temperature changes the long period significantly, but there is no significant difference in the crystallinity according to the treatment conditions.The content of the free volume in the untreated PET fibres is very low, as expected. Due to the physical treatment at the SCF conditions, some modifications of the microvoid system were observed.