Melt spinning of poly(L-lactic acid) and its biodegradability

The melt spinning and melt drawing of poly(L-lactic acid) (PLLA) were carried out with a melt-spinning machine, and the mechanical properties, structure, and biodegradability of PLLA fiber were investigated. PLLA fiber with a tensile strength of 0.81 GPa was successfully obtained through two steps of drawing at a draw ratio of 18 in hot water. This fiber had enough tensile strength for common engineering use. The fiber could be degraded under controlled composting conditions at 70°C for 1 week. In scanning electron microscopy observations of the fiber, a regular pattern of cracks running along the vertical direction to the fiber axis was clearly observed. This suggested that the PLLA fiber built up a highly ordered structure arranged along the direction of the fiber axis. After the fiber was left to lie in the ground for 1 year, however, the surface of the fiber was still smooth, and the tensile strength did not decrease much. This PLLA fiber could not be hydrolyzed after 1 month of steeping in a buffer solution at 37°C, but it was rapidly hydrolyzed at more than 60°C. It was suggested that the degradation (hydrolysis) rate of PLLA depended on the glass-transition temperature. Upon hydrolysis at 80°C for 48 h, a regular crack along the vertical direction to the fiber axis was found that was very similar to that observed in degradation under composting conditions.