In this paper, active species in a cold atmospheric pressure RF capacitive Ar plasma jet were characterized using optical emission spectrometry (OES). OES revealed that excited Ar atoms (4p-4s transitions) were predominant in the plasma jet, and no ions were detected in the plasma afterglow even after the addition of N₂ or O₂. It was expected that this Ar plasma jet was quite suitable for polymer surface treatment with no serious surface decomposition. As an application to the Ar plasma jet, high-density polyethylene (HDPE) surface was treated with Ar, Ar/O₂ and Ar/N₂ plasmas, respectively. Influences of additive gas on the improvement of the hydrophilicity of polymer surface were investigated in details. It was found that an enhancement of the hydrophilicity of HDPE surface was observed when N₂ or O₂ as an additive gas was added into the Ar plasma.

In this study, a high-density polyethylene (HDPE, 5-mm-thick, 0.95g/cm³) surface was treated using two types of cold atmospheric pressure Ar plasma jets (CAPPLAT plasma and APC plasma). In particular, the effects of the additive gas (N₂ or O₂) on the HDPE surface treatment were investigated and compared in details. Using the two Ar plasma jets, a large number of O-containing functional groups were introduced onto HDPE surface during the plasma treatment. However, the effects of the additive gas on the HDPE surface treatment are quite different from each other. In case of CAPPLAT plasma treatment, the water contact angle (WCA) value of treated HDPE surface increased slightly when trace of the additive gas was added into CAPPLAT plasma. However, in case of APC plasma treatment, the WCA value of treated HDPE surface decreased significantly when the same amount of the additive gas was added.