Study On Etching Of Polyamide 6 Films Treated With Atmospheric Pressure Plasma Jet

Plasma treatment is a gaseous technology which combines physical and chemical reactions. Compared with traditional physical and chemical treatments, it has advantages of low pollution and low energy consumption without involving water and chemicals. In low temperature plasmas, electrons with high energy and other excited or ionized particles initiate physical and chemical reactions only on the surface of the substrate with the thickness of several nanometers, leaving the bulk properties unchanged. Therefore, as an environmentally friendly surface modification technology, low temperature plasma treatments have been widely used to modify polymer surfaces.Recently more attention has been paid to atmospheric pressure plasma treatment due to advantages such as no need for a vacuum system, online process capabilities, high efficiency and scalability to a larger area. Plasma etching removes a weak boundary near the surface of the polymers or the low-molecular-weight fragments formed as a result of chain scission induced by the plasma. The active species in radio frequency plasma have the ability of breaking primary chemical bonds and inducing cross-linking. The plasma etching rate is related to the chemical structure of the polymer at a given plasma treatment condition. It is also greatly affected by the plasma treatment conditions such as treatment duration, output power, gas flow rate, jet to substrate distance and moisture regain. However, no systematic study has been reported on how these treatment conditions influence the etching rate.This research is aimed to employ the atmospheric pressure plasma treatment on polyamide 6 (PA 6) films to improve their surface properties including the enhancement of wettability, surface energy and T-peel strength. The change of the surface properties after atmospheric pressure plasma treatment and the etching effect of the treatment were studied and the mechanisms were discussed systematically based on the surface analysis method such as contact angle measurement, scanning electron microscope (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and T-peel strength.In this study, atmospheric pressure plasma is used to etch the surface of PA 6 films to investigate the etching behavior of PA 6 film surface by APPJ with helium/oxygen gases with different treatment time. The etching rate first increases and then decreases as the plasma treatment time increases. The hydrophilicity tests reveal the improvement of the hydrophilicity of the surface as the decrease of water contact angle measured after the plasma treatments, but the values do not change significantly with longer treatment time although slightly smaller values are observed for time in the 60-180 s range. AFM showed that the surface roughness increased after the plasma treatment. The deterioration of the surface morphology is more severe after a longer treatment time as rougher surfaces are observed due to the plasma etching effect. XPS results show a significant increase in oxygen content with the addition of carboxylic and hydroxylic groups and a decrease in the carbon content of the surface. It can be concluded that more oxygen containing polar groups are introduced after longer plasma treatment time.The influence of plasma treatment power on the atmospheric pressure plasma treatment is also investigated. The hydrophilicity tests reveal that the water contact angle decreases significantly after the plasma treatment. A higher treatment power results in a lower water contact angle. XPS results show a significant increase in oxygen content with the addition of carboxylic and hydroxylic groups and a decrease in the carbon content of the surface. AFM shows that the surface roughness and the weight loss also increase with the increase of plasma treatment power.To investigate the relationship between the etching effect and the gas composition of atmospheric pressure plasma treatment, pure helium, helium + 1% oxygen and helium + 2% oxygen are used as the working gases, the output power is 40 W, and the treatment time was 30 s. The He and He + O₂ plasma treated polyamide 6 films show increased surface roughness, surface oxygen contents and hydrophilic polar groups, leading to lower water contact angles, and higher T-peel strength than those of the control. When the amount of oxygen increases from 1% to 2% in the plasma gas mixture, all the above favorable effects are further enhanced. Plasma etching rate is promoted as the amount of oxygen in the plasma gas mixture increases. The T-peel bonding strengths of the plasma treated PA 6 films is raised steadily as the treatment time increases and among three types of gas mixture, He + 2% O₂ plasma has the highest bonding strength for the same duration of plasma treatment.PA 6 films are treated using atmospheric pressure plasma with different jet-to-substrate distance. Decrease in contact angle is observed under 2 mm or 3 mm of jet-to-substrate distance. However, the contact angle does not change when jet-to-substrate distance is 1 mm or 6 mm. It can be seen that the peel strength increases when jet-to-substrate distance is 2 mm or 3 mm, and the peel strength is the largest when jet-to-substrate distance is 2 mm. However, the peel strength does not change when jet-to-substrate distance is 1 mm or 6 mm. These results correspond to the SEM results. The etching rate increases first and then decreases as the jet-to-substrate distance increases. When the distance is smaller than 1 mm or larger than 6 mm, the plasma etching rate is almost zero. When the distance is 2 - 3 mm, the etching rate is the largest.The etching behavior of He/CF₄ atmospheric pressure plasma treatment to PA 6 film surfaces is also investigated. For a short treatment time, a decrease in contact angle and an increase in T-peel strength are observed corresponding to a relatively large increase in surface oxygen content and relatively small increase in surface fluorine content. However, as the treatment time increases further, the contact angle increases and T-peel strength decreases accompanied by a large increase in fluorine content and a relatively small increase in surface oxygen content. In addition, the surface roughness continuously increases and the plasma etching rate steadily decreases as the plasma treatment time prolongs. One of the main differences between a low pressure plasma treatment and an atmospheric pressure plasma treatment is that in atmosphere, the substrate material may absorb significant amount of water which may potentially influence the plasma treatment effects. The influence of the moisture regains of PA 6 film on plasma etching behavior is also studied. It is found that a higher moisture regain leads to greater surface roughness and a higher etching rate. Moisture absorbed into the film facilitated the plasma etching reactions on the PA 6 film surfaces, because the moisture in the film increases the amorphous area and breaking up the intermolecular bonds in the amorphous region.In this part, the effect of ethanol on the atmospheric pressure plasma treatment is investigated. The plasma directly treated sample has a significantly lower water contact angle than that of control while the ethanol pretreated sample has a water contact angle similar to that of the control. The surface of the plasma directly treated samples has the greatest roughness. Most of the oxygen containing polar groups increase (such as -C-O-, CONH and -COO-) after the plasma treatment. The T-peel strength increased after plasma treatment. However, with the ethanol pretreatment, the T-peel strength values for the samples are similar to that of the control. This is mainly due to the ethanol pretreatment quenches the etching effect of the films in plasma treatment.In conclusion, polyamide 6 films have been studied through surface analysis methods after atmospheric pressure plasma treatment. The treatment effect can be optimized and the etching effect of the atmospheric pressure plasma treatment can be promoted through selecting most suitable process parameters to optimize the material surface modification.