Morphology Gradients in Thermally Point-Bonded Poly(ethylene Terephthalate) Nonwovens

In a previous article, a sharp change in the birefringence and density of polypropylene fibers was reported at the edge of thermal point bonds in nonwovens. In this article, similar results are shown for polyester nonwovens. First, in both isotactic polypropylene (ipp) and poly(ethylene terephthalate) (PET), the morphology of the fibers between the bond points is identical to that of the original feed fibers. Second, the morphology of the bond points is substantially different from that of the fibers. Third, there is a rapid change in the morphology going from the fiber into the bond at the bond edge. Fourth, the birefringence of the fibers decreases by approximately 50% going from the fibers into the bonds. The change in birefringence is a function of processing conditions. In PET, the density in the bond point is lower than in the bridging fibers, while in ipp, the density in the bond point is higher than in the bridging fiberts. This difference is readily understood based on the different crystallization rates of ipp and PET.