Structure of poly(ethylene terephthalate) fibers treated in supercritical carbon dioxide fluid

The dyeing behaviors and changes in fiber structure in supercritical CO2 fluid for several types of high-speed spun and regular fully oriented poly (ethylene terephthalate) (PET) fibers were compared. At lower temperature and pressure, the high-speed spun fibers, which had inherently larger crystallite sizes and lower birefringence, showed a larger dye uptake than the other fibers. However, when the supercritical conditions were elevated to 125 degreesC and 23 MPa, the dye uptake of both types increased markedly and the difference in dye uptake between the fibers became small. This suggests that the swelling of fibers in supercritical CO2 fluid exceeded a certain degree and then the diffusion of dye molecules was promoted. The swelling also promoted the rearrangement of molecular chains. The combination of the high speed spinning technique and the supercritical CO2 fluid treatment can provide a new PET fiber with both larger crystallite sizes and higher molecular orientation in the amorphous regions.