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.