Discharge Energy Versus Exposure Time in Atmospheric-Pressure Air Plasma Surface Treatment of Polyimide and Polyamide 6 Films

Polyimide (PI) and polyamide 6 (PA6) films are treated under exposure times of 0.5 s and 1.0 s, and energy levels of 1.5, 2.0, and 2.3 mJ/pulse. PI exhibits the most substantial improvement in wettability and adhesion-related properties compared to PA6 and other studied polar polymers. The threshold level for stable surface modification is reduced, achieving a minimum water contact angle of 45°. The stability is markedly enhanced, with aged PI surfaces showing a 40% relative increase in adhesion work compared to untreated samples. The oxygen content on the PI surface reaches 22 at. %, surpassing the maximum of 18 at. % O observed for PA6. The surface roughness of PI increases by approximately a factor of 2, while PA6 shows an average increase of only 25%, attributed to higher ablation rates in the amorphous phase compared to the crystalline phase. The degree of surface modification achieved with [1.0 s; 1.5 mJ] treatment parameters is comparable to that with [0.5 s; 2.0 mJ], demonstrating that higher discharge energy can effectively shorten the required exposure time. This plasma treatment, even at very short exposure times, enables significant enhancement of the surface properties of PI, typically characterized by high chemical stability.