Improvement of mechanical properties of biodegradable PHBH fibers through high-speed melt spinning process equipped with a liquid isothermal bath

A biodegradable bio-based copolyester, poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) with the 3-hydroxyhexanoate composition of 5.4 mol%, was used in this study. A liquid isothermal bath (LIB) was installed into the high-speed melt spinning process to improving the mechanical properties of the PHBH fibers. When the spin-line passes through the LIB, temperature of the molten polymer can be controlled to the liquid temperature whereas the frictional stress of about 100 times higher than that in the air can be applied to the spin-line in the liquid. In the process with the LIB, the liquid temperature, the depth of the liquid bath and the take-up velocity were varied. Tensile strength of the high-speed spun PHBH fibers without LIB was only around 100 MPa. On the other hand, tensile strength of the fibers prepared with the LIB increased with the reduction of liquid temperature and with the increase of take-up velocity. At the take-up velocity of 1 km/min, liquid temperature of 40 °C and liquid depth of 20 cm, the fibers with the tensile strength of 176 MPa could be obtained. The WAXD measurement revealed that along with the highly oriented β-form crystals, co-existence of the β-form crystals was confirmed for the PHBH fibers of high strength prepared using the LIB.