Kyoung Hou Kim

Fine structure and physical properties of polyethylene/poly(ethylene terephthalate) bicomponent fibers in high-speed spinning. I. Polyethylene sheath/poly(ethylene terephthalate) core fibers

The high-speed melt spinning of sheath/core type bicomponent fibers was performed and the change of fiber structure with increasing take-up velocity was investigated. Two kinds of polyethylene, high density and linear low density (HDPE, LLDPE) with melt flow rates (MFR) of 11 and 50, [HDPE(11), LLDPE(50)], and poly(ethylene terephthalate) (PET) were selected and two sets of sheath/core combinations [HDPE(11)/PET and LLDPE(50)/PET bicomponent fibers] were studied. The fiber structure formation and physical property effects on the take-up velocities were investigated with birefringence, wide-angle X-ray diffraction, thermal analysis, tensile tests, and so forth. In the fiber structure formation of PE/PET, the PET component was developed but the PE components were suppressed in high-speed spinning. The different kinds of PE had little affect on the fine structure formation of bicomponent fibers. The difference in the mechanical properties of the bicomponent fiber with the MFR was very small. The instability of the interface was shown above a take-up velocity of 4 km/min, where the orientation-induced crystallization of PET started. LLDPE(50)/PET has a larger difference in intrinsic viscosity and a higher stability of the interface compared to the HDPE(11)/PET bicomponent fibers.

Visualization of a pillar-shaped fiber bundle in a model needle-punched nonwoven fabric using X-ray micro-computed tomography

In the needle-punching process, the barbs of a needle catch fibers and orient them along the thickness direction of the fabric. The oriented fibers form a pillar-shaped fiber bundle, which acts as a bonding point of the fabric. The structure of the pillar-shaped fiber bundle thus governs the mechanical properties of needle-punched nonwoven fabric, and both are largely affected by the needle-punching conditions. However, the three-dimensional structure of pillar-shaped fiber bundles and their development under different needle-punching conditions have not been revealed. In the present study, we visualized the three-dimensional structure of a pillar-shaped fiber bundle in needle-punched nonwoven fabric, employing X-ray micro-computed tomography (XCT) on the basis of the difference in the X-ray absorption coefficient between polyethylene terephthalate (PET) and polyethylene fibers. For a material density ratio of less than 1.4 and PET fibers having a diameter of 40 µm, the pillar-shaped bundles of PET fibers were visualized by erasing 20-µm polyethylene fibers in XCT images. Furthermore, we investigated the effects of the penetration depth of the needle on the development of pillar-shaped fiber bundles. The number of fibers constituting a pillar largely increased at a penetration depth of 19.0 mm, and pillars protruded from the bottom surface of the fabric and formed a stitch structure. The XCT applied in this study is thus effective in analyzing the structure of pillar-shaped fiber bundles quantitatively without affecting the structure of the nonwoven fabric.

Fine structure and physical properties of polyethylene fibers in high-speed spinning. II. Effect of catalyst systems in linear low-density polyethylene

Linear low-density polyethylene (LLDPE) fibers, obtained from the melt-flow rate (g/10 min) of 45 and 50, which were polymerized by a metallocene catalyst and a Ziegler–Natta catalyst, respectively, were produced by a high-speed melt-spinning method in the range of take-up velocity from 1 to 6 km/min. The change of the fiber structure and physical properties with increasing take-up velocity was investigated through birefringence, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), a Rheovibron, and a Fafegraph-M. The birefringence increased linearly with increasing take-up velocity and that of LLDPE(45) was higher than that of LLDPE(50). With increasing take-up velocity, the crystal orientation of LLDPE transformed the a-axis orientation into a c-axis orientation. In the dynamic viscoelastic behavior of LLDPE(45) fibers with high-speed spinning, the intensity of the crystalline relaxation peak was decreased and the temperature of that was shifted lower. But that of LLDPE(50) could not be observed. The tensile strength and initial modulus were increased and the elongation was decreased with increasing take-up velocity. LLDPE(45) fibers were preferred to LLDPE(50) in mechanical properties owing to the increase of crystal and amorphous orientation factor. The change of birefringence with take-up velocity affected both the initial modulus and the tenacity uniformly.

Microstructural analysis of melt-blown nonwoven fabric by X-ray micro computed tomography

The structures of melt-blown nonwoven fabric, including the fiber volume fraction and fiber orientation, are decided by the melt-blowing conditions, such as the die-to-collector distance (DCD), air suction and air flow rate. In this study, the effects of these melt-blowing conditions on the structure of melt-blown fabric was investigated by X-ray micro computed tomography with a resolution of 1 µm/voxel and a measurement area of 1 mm2. The structural profile along the thickness direction of the fabric was also analyzed. Obtained averages of fiber diameter and basis weight were almost identical to the results of scanning electron microscopy measurements and tests following the standard of International Organization for Standardization. Samples produced with insufficient air suction showed larger deviation of basis weight on the 1 mm2 scale compared with those produced with sufficient air suction. The fiber volume fraction changed steeply around the fabric surface, which was attributed to surface roughness. Insufficient DCD increased the fiber diameter and surface roughness, and decreased the thickness of the fabric. The fiber volume fraction gradually decreased from the collector side to the die side for fabrics with sufficient DCD. The fibers were oriented along the machine direction rather than the cross-machine direction for all layers and samples, and orientation profiles corresponded to the zero-span tensile strength of corresponding samples.

Response of fiber diameter to a continuously changing draw ratio in a laser drawing process

The controllability of the fiber diameter profile with a changing draw ratio in a laser drawing process is discussed in comparison with model calculations. The take-up speed in the continuous drawing process was changed under constant acceleration, and the diameter profile of the resultant fiber was monitored on-line. It was revealed that the diameter profile was consistent with a profile predicted by a model calculation up to a certain acceleration. The profile was calculated by assuming a stationary neck-drawing point and elastic deformation of the fiber in the drawing system. Video observation revealed that the neck-drawing point did in fact move with changes in the draw ratio. Moreover, it moved faster at higher accelerations, particularly when the draw ratio was increased from low to high. It seems that the faster movement of the neck-drawing point causes the observed diameter change to lag behind that predicted by the model. The responsiveness of the diameter change could be further improved by shortening the distance between the feed and take-up rollers. The minimum response length was 5 cm as the draw ratio decreased from 8.0 to 4.0.

Fabrication of Hollowed Polytrimethylene Terephthalate Fiber by Carbon Dioxide Laser Irradiation

ABSTRACT Polytrimethylene terephthalate hollowed fiber was fabricated by neck drawing process with carbon dioxide irradiation. Fiber diameter increased with increasing draw ratio, and the hollowed fiber of hollowness ratio 46.4% was formed with draw ratio 5.5. Scanning electron microscopy showed internal multiple hollows was formed in fiber cross-section. Drawing stress obtained from drawing tension in situ monitored during the drawing increased with draw ratio, and it corresponded to the increasing trend of hollowness ratio. Necking images showed that the fiber diameter decreased steeply with increasing draw ratio, and the drastic decrease with high draw ratio generated the multiple hollows. GRAPHICAL ABSTRACT

Effect of Vibration on Drawing and Annealing of High- Speed Spun Poly(trimethylene terephthalate) Fiber

Abstract The physical properties of poly(trimethylene terephthalate) (PTT) fibres were improved by means of vibration in the hot-drawing and annealing, which may be caused by the developed molecular packing. For high-speed spun PTT fibers, it was not until at the take-up speed of 3~4 km/min that the orientation induced crystallization started to emerge due to extensional stress occurred in spin line; confirmed from the results of WAXD and DSC. The PTT fibers obtained at the take-up speeds of 2~3 km/min and then drawn and annealed with vibration possessed low density and weight-crystallinity, but their birefringence was especially high. Moreover, the estimation of both refractive index parallel and normal directions to fiber axis using the interference microscopy showed that the refractive index parallel to the fiber axis was very high, which enhanced the mechanical properties of PTT fiber. Accordingly, the well-oriented chains along the fiber axis allow the PTT fiber to have better physical property such as elastic recovery although the PTT fiber has low density and crystallinity compared to PET and PBT. In effect, the PTT fiber possesses lower birefringence of over 10 times than those of PET and PBT due to its chain conformational characteristics. Therefore, we do suggest that the structural assessment against the subsequent mechanical properties according to various processes in the PTT fiber is preferred to be estimated through the respective refractive indices of parallel and normal to the fiber axis rather than conventional methods such as birefringence, crystallinity, and crystalline orientation.

Fine-structure and physical properties of polyethylene fibers in high-speed spinning. I. Effect of melt-flow rate in the high-density polyethylene

High-density polyethylene (HDPE) fibers, obtained from a melt-flow rate (g/10 min) of 11 and 28, was produced by a high-speed melt-spinning method in the range of take-up velocity from 1 to 8 km/min and from 1 to 6 km/min, respectively. The change of fiber structure and physical properties with increasing take-up velocity was investigated through birefringence, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), a Rheovibron, and a Fafegraph-M. With an increase in take-up velocity, the birefringence showed a sigmoidal increase, which has distinct changes in the range of 1–5 km/min. Throughout the whole take-up velocities, the birefringence of HDPE(11) was higher than that of HDPE(28). With increasing take-up velocity, the crystalline orientation was transformed from a-axis orientation to c-axis orientation. These crystalline relaxations are confirmed by the tan δ peak of high-speed spun HDPE fibers. The intensity of the crystalline relaxation peak decreases with increasing take-up velocity in both HDPE(11) and HDPE(28). As above, the crystalline relaxation peaks shift to lower temperature with increasing take-up velocity. With increasing take-up velocity, the ultimate strain decreases while both specific stress and the initial modulus increase. The mechanical behavior may be closely related to, as investigated by birefringence, orientation of the amorphous region, etc., the take-up velocity.