| Based on the modified one-step FDY(full draw yarn) producing equipment and guided by the formation theory of macromolecules structure in high-speed spinning process, this paper studied the spinning mechanism of one-step BSY (bi-shrinkage yarns) and optimized the spinning technology. The BSY were intermingled by high-shrinkage filaments (low crystallization pre-oriented yarn) and low shrinkage filaments (with high crystallization and orientation of tensile filaments). The two components went through spinnerets and cool air respectively. At a particular cluster point, the two parts were combined into a bunch of filaments and then shaped by winding. The structure and properties of BSY were determined by the super-molecular structure, physical and mechanical properties and the proportion of its two components. Especially, the differences of shrinkage between the two components affected the mechanical properties and fluffy degree greatly.This paper focused on one-step BSY spinning process and the effect of spinning parameters to the structure and properties of BSY, analysis controlling and adjusting mechanism of difference shrinkage between components, optimize one-step BSY the spinning parameters.when take-up velocity fixed at 3000m/min, through optimization of the experiment plan, we obtain many BSY samples under the conditions of different draw ratio, tension-setting temperature. In this paper, the strength, elongation at break, modulus and other indicators are tested by electronic single yarn strength tester; optical microscope.The morphology structure, crystallinity, the sonic velocity orientation factor, boiling water shrinkage, velocity modulus are tested and analyzed by 3D video microscope, DSC, the digital fiber sound velocimeterand so on.Research and analysis found that:(1) The GR2 speed V2 fixed, the crystallinity decreased gradually with the GR1 speed V1 increasing from 650m/min to 950m/min; Orientation degree of BSY increases with the GR1 speed V1 increasing from 650m/min to 740m/min, and when V1 continues increase, orientation degree of BSY decrease .This phenomenon is attributed to the reduction of FDY orientation degree because the post-drawing ratio decrease. Breaking tenacities and initial modulus decreases, boiling water shrinkage of BSY are increases,and the breaking elongation increases sharply.(2) The other conditions remain unchanged, with the GR1 temperature H1 increasing from 80℃to 95℃, crystallinity, breaking tenacities, initial modulus of BSY increase gradually, but its orientation reduce. The boiling water shrinkage decrease from 25.30% to 29.05% ,when H1 increase from 80℃to 85℃, And when H1 reach 95℃, the boiling water shrinkage decrease slightly.(a) With the GR2 temperature H2 increasing from 100℃~120℃, crystallinity of BSY increases, but orientation of BSY increases first and then decrease, its breaking tenacities decreases gradually, but not obvious, its boiling water shrinkage decreases gradually, and the boiling water shrinkage reduces to 29.5%, when H2 increases to 120℃. With the GR2 temperature H2 increasing from 100℃~ 115℃, its elongation at break decreases gradually,And initial modulus increase.But when H2 rises from 115℃to 120℃, elongation at break increases, and its initial modulus decreases.(b) After one-step BSY spinning process parameters optimization,single-screw different spinneret melt-spinning process could obtain high-quality BSY. When V1 in 740~900m/min, H1 at 85℃, H2 is set at 110℃, take-up velocity at 3000m/min, BSY comprehensive physical and mechanical properties like initial modulus, breaking tenacities, breaking elongation, boiling water shrinkage, difference boiling water shrinkage between components are excellent.(c) BSY tensile curve has two peaks. The two components do not break at the same time, FDY break first, and then POY break. The tensile process changed after the treatment in boiling water.(d) The transforms of BSY before and after the treatment in boiling water: The change of BSY breaking tenacity is not obvious and BSY breaking tenacity is decided by FDY; BSY breaking elongation increase sharply; BSY initial modulus decrease sharply, and is decided by POY.
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