Morphology Gradients in Thermally Point-Bonded Polypropylene Nonwovens

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Abstract
Development of higher speed processes in thermal point bonded nonwovens and of models of the strength of these nonwovens are hampered by the lack of quantitative knowledge of the morphology of the fibers, the bond points, and the transition region between them within the nonwoven. The complex structure of these nonwovens has rendered useless most analytical techniques for measuring morphology. In this work, we use a recently developed method of polarized laser Raman microspectroscopy to measure the morphologies of these regions quantitatively. There are several surprising results as well as some expected ones. First, the morphology of the fibers between the bond points is identical to that of the original feed fibers. Second, there is a sharp gradient in the morphology going from the fiber into the bond. Third, the morphology of the bond points is substantially different from that of the fibers. Fourth, the morphology change within the bond points is directly related to the bonding conditions. In the case of an overbonded nonwoven, the birefringence decreases by almost 70% over a distance of only 30 μm.