| As one of the renewable polymer materials, poly(lactic acid)(PLA) has good biocompatibility and biodegradability. It can be used in medical science, textile materials, as well as the agriculture area. However, the melting temperature of PLA is not high enough to achieve good thermal stability. The PLA fabric can not be dyed with conventional method, and then it also can’t be ironed, which increases the cost of PLA fabric finishing and restricts the application of PLA fibers. In foreign countries, PLA fibers with high content of stereocomplex crystallites can be formed by poly(L-lactic acid)(PLLA) and poly(D-lactic acid)(PDLA) blends melt spinning, drawing and annealing of PLA fibers at high temperatures. With this method, the melting temperature of PLA fibers increased, but high temperatures led to the lower orientation and even degradation of PLA fibers, thus the molecular weight and mechanical properties of PLA fibers decreased.In this paper, the blend system of PLLA, PDLA and dfifferent kinds of nucleating agents(TMP-5, TMC-328, HT) was melt spun into fibers. The effects of nucleating agents on the structure and properties of the fiber were studied. The results indicated that the blend melt spinning of PLLA and PDLA can form stereocomplex PLA (sc-PLA) crystal which melting temperature could be above205℃. Thus the thermal-resistance of PLLA/PDLA blend fibers was greatly enhanced. The formation of sc-PLA crystal in the blend fibers was increased but the mechanical properties of the blend fibers were decreased differently by the addition nucleating agents. Moreover, the comprehensive effect of the nucleating agent TMP-5was best because it not only improved the thermal property, but also relatively kept good mechanical properties of the blend fibers.It was significant to study the rheological properties of PLA melt blends for selecting the proper spinning processing technics. The blends of PLLA and PDLA were melt and made into slices in round shape. The rheological properties of melt blends were investigated by means of advanced rheometric expansion systems (ARES). The results showed that PLLA/PDLA melt blends were the typical kinds of pseudo plastic fluids and had high values of viscous flow activation energy, which indicated that the apparent viscosity was sensitive to temperature. Therefore, it must be strictly controlled the temperature in the spinning process. After different content of nucleating agent TMP-5added into the melt blends, the spinnability of PLA melt blends decreased. The melt blends were more suitable for spinning when the content of TMP-5was0.5%.Based on above results, the appropriate spinning process parameters were selected. Different contents of nucleating agent TMP-5were added to the blend system of PLLA/PDLA for melt spinning. After that the structure and properties of the blend fiber were studied. The results showed that the content of sc-PLA crystal in the blend fibers increased with the increasing content of nucleating agent TMP-5, which enhanced thermal-resistance of blend fibers. From the SEM photos, it was apparent to found that the nucleating agent TMP-5formed agglomeration when the content of nucleating agent TMP-5was0.7%, which affected the nucleating effiency. Under the same drawing condition, the orientation of PLA fibers was little changed. When the content of nucleating agent TMP-5was0.5%, it was achieved maximal breaking strength of the blend fibers.In the end, thermal property, mechanical properties and the structure of blend fibers were analyzed after different drawing and annealing process of as-spun fibers which the content of nucleating agent TMP-5was0.5%. During the one-step and two step drawing process, the results showed both processes had little effect on the thermal-resistance of blend fibers. As the one-step drawing ratio increased, the mechanical properties of blend fibers improved. When the total drawing ratio was the same in two-step drawing process, the mechanical properties of blend fibers improved slightly as the first drawing ratio increased. On the whole, blend fibers had a better mechanical performance in two-step drawing process. After the annealing of drawn fibers in fixed length at different temperatures, the total crystallinity of blend fibers increased. While elevating the annealing temperature, the formation of sc-PLA crystal in blend fibers increased gradually. When the annealing temperature was above180℃. there was only sc-PLA crystal in blend fibers. This indicated that the annealing temperature had a significant influence on the thermal-resistance of the blend fibers. However, the mechanical properties of the blends fibers reduced while the annealing temperature increased. In further study, the blend fibers were annealed for different periods and under different tensions at a certain temperature. Either changing the annealing periods or tensions had little effect on enhancing the formation of sc-PLA crystal and thermal-resistance of blend fibers. Nevertheless, the mechanical properties of blend fibers decreased when elevating the annealing periods. For this reason, the annealing period should not be too long. |
PDF Full Text Request