An attempt was made (1) to investigate a change in supermolecular structure of Poly (ethyleneterephthalate) (PET) fibers spun at high wind-up speed during heat treatment at high temperature for shortperiod of time (t_a) and (2) to clarify a change in supermolecular structure of PET fiber annealed and thenfalse-twisted. As PET fibers were prepared at spinning speed in the range 3∼4 km/min and subsequentlyannealed during 240∼270°C for a short period t_a (≤0.8 sec) of drawing (draw ratio 1 .0-1.4) at temperature240∼270°C . After heat treatment the fibers were false-twisted. PET fibers, annealed and false-twisted afterannealing were characterized by X-ray diffraction, temperature dependence of dynamic viscoelasticity, dyeexhaustion, tensile strength and tensile elongation.When as-spun fiber is heat-treated at high temperature T_a, both crystallinity and crystal size increase by false-twisting. As the heat treatment proceeds the amorphous regiononce changes to more dense structure with lower dye exhaustion, but further heat treatment relaxes themolecules in the amorphous region, resulting in an increase in the region having lower molecular packing density.This brings about an enhanced dye exhaustion and finally PET fibers dyeable under atmospheric pressurecan be obtained for t_a< 1sec. The dyeability of these PET fibers does not decrease even after false-twisting.Change in the peak value and the peak temperature of tan o is well consistent with that of dye exhaustionbehavior. The tensile strength increases and the tensile elongation decreases by heat treatment . The tensilestrength does not change, but the tensile elongation decreases further by false-twisting, which gives PETfibres having suitable tensile properties and dyeability under atmospheric pressure for pratical use .

View full abstract

To find out the cause of decrease in tightening tension and increase in the variation of the tension whichoccurs frequently during the high-speed sewing operation on an industrial single-needle lockstitch sewingmachine, this paper aims, in succesion to the previous paper (1) to investigate experimentally the relation betweenthe length of needle thread drawn out of a tension disk when the needle thread disengages from rotatinghook and tightening tension, and to discuss about the elongation properties of sewing thread with respect tothe length of needle thread when the needle thread disengages from rotating hook.
For this purpose two measuring systems were developed. One is a system, which was improved system usedin the previous paper (1), to observe photographically needle thread behaviors when the needle thread isgoing to disengage from rotating hook and to measure the tension of the thread generated at the same time.
The other is a system to measure the length of needle thread tension.
The results obtained are as follows :
(1) It was confirmed experimentally that the needle thread was drawn out of a tension disc not only whenthe thread tightened a stitch but also when the thread was disengaged from rotating hook, and thatthe more the drawn length was long, the more tightening tension was decreased.
(2) It was supposed that disengagement behaviors of needle thread from rotating hook were decided bythe length of the needle thread, and that the length depends entirely on the elongation recovery propertyof the sewing thread.

View full abstract