Study On Preparation,Structure And Properties Of Stereo Composite Polylactic Acid Fiber

Polylactic acid(PLA)is a kind of green fiber with complete biodegradability and sustainable development,but its application range is greatly limited by its poor temperature resistance and hydrothermal degradation resistance.The stereo complex structure(Sc)can be formed between L-polylactic-acid(PLLA)and D-polylactic-acid(PDLA).The melting point of PLA with this structure will be increased by about 50?and its biodegradability will not be affected.In terms of efficiency,environmental protection and other factors,melt blend spinning of PLLA and PDLA is the optimal method for the preparation of ScPLA fiber.However,the key melt dispersion part of PLLA/PDLA system has not been deeply studied.In addition,the growth of the stereo composite structure of the fiber often requires a long time of high temperature treatment,which is difficult to meet the requirements of modern industrial preparation of ScPLA fiber.Therefore,by studying the rheological and thermal properties of PLLA and PDLA mixed in different ways,this paper explores the mixing method which is more suitable for melt spinning to prepare ScPLA fiber,and the ScPLA fiber is prepared by spinning-drawing two-step method.The changes of thermal properties,crystallization properties and orientation properties of ScPLA fiber with preparation conditions were studied,and the law of Sc crystal formation and the time-temperature evolution of Hc crystal and Sc crystal during the preparation of ScPLA fiber were clarified,which provided some theoretical support for the industrial preparation of ScPLA fiber.First of all,the thermal properties of PLLA slices,PDLA slices and screw mixed granules were tested by differential scanning calorimeter,and the rheological properties of room temperature dry blending and screw mixed granulation were further studied.The thermal performance analysis shows that a small amount of cubic composite crystals are formed in the screw mixture,and the melting point of PLLA and PDLA chips is about 175?.Because the room temperature dry blending method is difficult to mix PLLA and PDLA evenly,the melt fluidity and flow stability fluctuate below 240?,and the melt apparent viscosity and flow stability of the screw mixture are extremely sensitive to the change of temperature,which is not allowed to fluctuate above 2?3?when used in spinning.secondly,the fully stereo composite polylactic acid containing only stereo composite crystals was prepared,and its melting point reached 234?.The non-isothermal crystallization kinetics of fully stereotactic poly(lactic acid)was analyzed by Jeziorny method and Mo Zhishen method.The Avrami index is between 1?2 when the cooling rate is 5?/min,indicating that the crystal grows in one-dimensional and two-dimensional form,and when the cooling rate is lower than 1?/min,the Avrami index is close to or even more than 4,indicating that polylactic acid is mainly homogeneous nucleation and crystal growth in three-dimensional form.The rheological properties of fully stereotactic polylactic acid composite showed that its fluidity and flow stability were obviously lower than those of room temperature dry blends and screw blends,and the dependence of melt apparent viscosity on shear rate was greater than that of temperature when the shear rate was higher than 3300s^-1.summarize the feasibility of room temperature dry blending,screw mixing granulation and stereotactic composite polylactic acid method for spinning ScPLA fiber,and comprehensively analyze the rheological properties of the three blending methods and the crystallization properties of stereotactic composite polylactic acid.finally,based on the above research on rheological and crystallization properties,trial spinning was carried out by metering blending with equal mass of metering pump,and PLLA/PDLA mixed melt was obtained after metering and blending,and then ScPLA preoriented fiber was successfully prepared by high speed melt spinning,and PLLA and PDLA were fully dispersed in the melt.However,because curing is a rapid cooling process,the crystallinity of ScPLA preoriented fiber is very low.ScPLA draft fiber was prepared under fixed draft ratio and thermal drawing temperature and different heat setting temperature and time.Through the analysis of crystallinity,it is found that most of the Sc crystals in the ScPLA draft fibers are formed by the transformation of Hc crystals in the process of heat setting.The fully stereotactic composite fibers are obtained at temperatures above 177?,and the Sc content can reach 60%at 197?.In the heat setting temperature range of165C?177?,the orientation degree of the fiber decreases sharply due to the disorientation in the amorphous zone and the destruction of Hc crystals.For the ScPLA draft fiber with 1 time heat setting time,the breaking strength is about 1.8c N/dtex in the setting temperature range of 177?197?,while that of ScPLA draft fiber treated with 3 times heat setting time is 177?197?,and the fracture strength can be kept above 1.9 c N/dtex.The boiling water shrinkage of the fiber decreases gradually with the increase of the heat setting temperature,indicating that the thermal stability and dimensional stability of the fiber are significantly enhanced with the increase of the heat setting temperature.