Study of melt spinnability and drawing process of ethylene vinyl alcohol (EVOH) copolymers

Autor:	Zhi-Feng Jue, 角志峰
Druh dokumentu:	學位論文 ; thesis
Popis:	104 This study investigates melt spinnability of ethylene vinyl alcohol (EVOH) copolymers with various ethylene contents, which exhibit excellent biocompatibility but cannot easily undergo single-component fiber spinning. The chemical, thermal and rheological properties of EVOH were examined herein. Three EVOH copolymers, EV-32, 38, and 44, with different ratios of ethylene to vinyl alcohol (EV ratio), were used to evaluate the influence of the ratio on melt spinnability. The EV ratios of EV-32, EV-38, and EV-44 examined by NMR were 0.64, 0.82 and 1.0, respectively. The experimental results reveal that the melting temperature (Tm) and crystallinity (Xc) of EVOH decreased in the order, EV-32 (178.9℃, 46.6 %) > EV-38 (171.9℃, 41.8 %) > EV-44 (166.0℃, 40.0 %). The thermal stability, however, increases in the order EV-32 < EV-38 < EV-44. The viscosity decreases in the order EV-32 > EV-38 > EV-44. EV-44 has much higher flow activation energy than EV-32 and EV-38, indicating that it has a temperature sensitivity higher than EV-32 and EV-38. The pellets of EV-32, EV-38 and EV-44 were melt spun. Three as-spun fibers, EV-32, EV-38 and EV-44 have tensile stresses (strain (%) at breaking point) of 91.6 (205.2 %), 87.4 (202.0 %), and 95.9 MPa (180.2 %), respectively. Most importantly, EV-44 can easily be spun at relatively low temperature, 245℃, while the other two, EV-38 and EV-32, can hardly be made on condition that the spinning temperature is higher than 255 and 265℃, respectively. Notably, once the spinning temperature of EVOH polymers was higher than 258℃, the degradation of VA segments would cause fuming and broken filaments to eventually terminate the entire spinning process. This study investigates the effect of drawing process of ethylene vinyl alcohol (EVOH) fibers on their physical properties. Three different ethylene contents, namely EV-32, EV-38 and EV-44, were used where the ethylene content has the order of EV-44 > EV-38 > EV-32. The result indicates that at the same drawing temperature and draw ratio, the online drawing stress of the fiber with high ethylene content is higher than that with low ethylene content. Moreover, the drawn EVOH fiber, at the drawing temperature of 80℃and the draw ratio of 2.0, exhibits an optimal mechanical property. As the draw ratio increases, the online drawing stress, birefringence and initial modulus increase without surprising. Notably, unlike typical polymeric fibers, the glass transition temperature (Tg) of the drawn EVOH fibers decreases with the draw ratio due to more water being absorbed by thinner fibers within the same amount of samples. The draw ratio was found to have little effect on the melting temperature (Tm). At the same draw ratio, the online drawing stress, birefringence, stress and initial modulus of the fiber EV-44, which has the highest ethylene content, is higher than those of EV-32 and EV-38. The creep strain of the drawn fibers EV-32, EV-38 linearly increase with the drawing time when the applied stress maintains constant, 150 MPa, while insignificant increase is observed for EV-44, suggesting that EV-44 is difficult to deform and has higher size stability. In the stress relaxation test, the elongation increases with the initial stress. At the same elongation %, the initial stress of the drawn fibers has the following trend: EV-44 > EV-38 > EV-32 and the stress relaxation time (τ) has the following trend: EV-44 > EV-38 > EV-32, indicating again that EV-44 is relatively difficult to deform during drawing. As the elongation is increased, the elastic recovery of the drawn fiber is decreased. In the hot water shrinkage experiment indicates that the hot water resistance of EV-44 is the best among the three fibers.
Databáze:	Networked Digital Library of Theses & Dissertations
Externí odkaz:	http://ndltd.ncl.edu.tw/handle/ata9cs