Effect of absorbed moisture on the atmospheric plasma etching of polyamide fibers

A potential problem for the atmospheric pressure plasma treatment is that the moisture absorbed by the substrate may influence plasma surface modification processes. This study evaluated the effect of moisture regain on the surface morphology change of polyamide fibers by plasma etching. Polyamide 6, poly(p-phenylene terephthalamide) (PPTA, aromatic polyamide), wool (polyamide 2), and ultrahigh modulus polyethylene (UHMPE, polyamide infinity) fibers, were selected to represent various polyamide molecular structures. The fibers were plasma treated at three moisture regains corresponding to three different relative humidity levels (10, 65, and 100%). Scanning electron microscope (SEM) showed that no apparent morphology change was observed on the surface of UHMPE and PPTA fibers. Under the nano-scale surface analysis of atomic force microscopy (AFM), however, rougher surface of UHMPE and PPTA fibers appeared with elevated relative humidity or higher moisture regain. In terms of polyamide 6 and wool, SEM images revealed that compared to the slight plasma etching effect of fibers with the lowest moisture regain, a thin surface layer of the treated fibers with higher moisture regain was partially or completely peeled off. It may be concluded that fiber moisture regain plays an important role in atmospheric pressure plasma etching of polyamide fibers, which may be mainly due to the interaction between the absorbed water and the polymer molecules. It can be concluded that the etching rate of atmospheric pressure plasma for a polymer depends on its moisture regain, intermolecular forces, crystallinity, and molecular structure.