Summary: Spin‐drawing yarn (SDY) and high‐speed spun (HSS) fibers were produced from recycled poly(ethylene terephthalate) pellets, and the oligomer deposition on the surface of fiber in supercritical carbon dioxide fluid was compared. The oligomer deposition was obvious for SDY, which corresponded to the fact that the amount of imperfect small crystallites generated in amorphous regions through s‐CO2 treatment was much smaller for SDY as compared to that with HSS. The imperfect small crystallites generated in amorphous regions will work as a barrier to the oligomer migration. In order to suppress the deposition of oligomer for SDY, the combination of a heatset treatment and an alkaline etching was useful. At a heatset process, the oligomer migration to the surface layer tends to occur, and the layer becomes rich in oligomer. The following topochemical hydrolysis reaction at alkaline etching removes such layer. Accordingly, the total content of oligomer in etched fiber is reduced. As a result, the oligomer deposition in s‐CO2 treatment becomes not so serious. In addition, the heatset treatment gives the shrinkproof property to SDY, and the property is still retained after the alkaline etching.
