| High performance is an important direction in the development of polymer materials. In the field of fiber, high performance fiber has been hot topic for many years. Nylon is a kind of typical crystalline polymer, with many better properties than those of polyester fiber. In order to improve the mechanical properties of PA6fiber by high ratio drawing, many researchers have made in-depth exploration on the modification of nylon6. But most researchers didn’t get fiber with high ratio drawing, thus they couldn’t get nylon6fiber with good mechanical properties. For amide groups between adjacent nylon chain will form hydrogen bonds, the molecular inter-atomic force is big and molecular chain arrangement is neat, leading to its tendency to form crystals, and this is certainly not conducive to high ratio drawing.It was reported that the hydrogen bonds between the molecular chains could be destroyed through the interaction between Li+and amide groups, and this would benefit the high drawing and increase the mechanical properties of the PA fiber. There were relevant reports about the method of modifying nylon6by LiCl. For example, they prepared LiCl additive masterbatch through twin screw extrusion, and the masterbatch was mixed with nylon6chips and melt spun to get nylon6fibers with good properties. But the preparation process was complex and consumed much energy. Meanwhile LiCl has strong water absorption ability, and it could absorb water easily when exposed to the air in the process of blending. The screw extrusion blending method didn’t solve this problem. Besides, it takes long time to remove the water of mixture in the vacuum at high temperature, and it was not easy to keep the slice dry for a long time after drying. If we want to improve fiber properties by adding LiCl, we must solve this serious problem. With the purpose of increasing its draw ratio and mechanical properties, in this thesis, we used LiCl as hydrogen bond inhibitor, which was cladded by nylon6chip at varying proportions by high-speed cladding methods, and then melt spun to get LiCl/PA6fibers. After hot drawing and washing with hot water to remove the LiCl and nylon6monomer, the final PA6fiber was obtained. The effect of different proportion of LiCl on thermal properties, rheological properties and crystallinity of nylon6particles were studied; the influence of hot melt spinning and drawing process on crystal properties, orientation structure, thermal properties and mechanical properties of LiCl modified PA6fibers were also explored. The mechanical properties and morphology change of PA6fiber after water washing were also discussed in this paper.The results of DSC and melt flow index test showed that, as the LiCl content increased, the crystallinity, melting temperature and crystallization temperature reduced, but melt viscosity increased gradually. FTIR demonstrated that the reasons for the above experimental results might be that lithium ion had formed the six-member rings structure with nylon6chains, and this destroyed the hydrogen bonds between the nylon6molecular chain and the original neat arrangement. Apparent viscosity of different ratio of LiCl/PA6was studied by capillary rheometer at various shear rates, and the results showed the "shear-thinning" phenomenon. Non-newtonian index was between0.811～0.846, as the content of LiCl increased, non-newtonian index decreased.For both pure PA6and LiCl/PA6, non-isothermal crystallization kinetics study showed that starting crystallization temperature and crystallization peak temperature decreased with the increase of cooling rate, and there was secondary crystallization process. And after adding0.5%LiCl, the starting crystallization temperature, crystallization temperature and crystallinity of PA6were lower than those of the pure PA6. Mo Zhishen method could be used to describe the non-isothermal crystallization process of PA6very well, and F(T) increased with the increase of relative crystallinity.DSC, TGA, SEM and acoustic velocity and mechanical properties testing results showed that when the LiCl content was0.5%, the fiber axial orientation degree and draw ratio were the largest, fiber surface was smooth, and heat resistance and mechanical properties was the best. When the spinning speed was800m/min, hot plate temperature was60℃, and hot plate temperature was120℃, the draw ratio could be as high as4.8times. The fiber diameter was14.65μ m, the crystallinity was37.22%, the fracture strength was5.3cN/dtex and elongation at break was5.8%. Compared with the pure PA6fiber, its breaking strength increased by112%, the elongation at break decreased by74.6%, and the fiber had the best comprehensive mechanical properties.DSC, FTIR and mechanics test showed that, after washing the uncombined LiCl was removed by water washing, and the undestroyed complexation was gradually removed and hydrogen bond was formed again between macromolecular chains. Melting temperature, melting enthalpy, crystallinity and fracture strength of fiber increased as the time of washing increased, while elongation at break decreased. The0.5%LiCl/PA6fiber was drawn to4.8times, and its fracture strength was5.8cN/dtex, elongation at break was5.0%after washing. and compared with the samples before washing, the breaking strength increased by8.6%, elongation at break decreased by13.8%, and comprehensive mechanical properties improved. But too long washing time wasn’t good, the best washing time was1h. The crystal structure of fiber changed after washing by boiling water. The results also showed that it appeared a kind of metastable β crystal type in the process of fiber spinning processing besides the y crystal type. The metastable β crystal type could transform into a stable a crystal type after being processed by boiling water. |
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