Atmospheric pressure dielectric barrier discharges for the surface modification of polypropylene

Seidelmann, Lukas Josef Wilfried (2015) Atmospheric pressure dielectric barrier discharges for the surface modification of polypropylene. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Polypropylene films are widely used as packaging materials for foodstuffs, since they offer excellent barrier properties against polar substances, high elasticity, mechanical strength, transparency and chemical stability. However, the relatively low surface energy of polypropylene represents a major issue for the adhesion of printing inks and labels. To improve printability it is necessary to perform a surface activation process. In the presented work, the atmospheric pressure dielectric barrier discharge (DBD) in nitrogen is investigated as a surface treatment method to increase the surface energy of biaxially orientated polypropylene foils. This technology creates a non-thermal plasma by applying strong electrical fields to a gas between two electrodes. The reactive species of the plasma forms new electronegative chemical groups on the surface and also etches the surface of the substrate. Both effects lead to an increase of the surface energy of the plasma treated polymer films. The crosslinking of polymer chains is also a possible effect caused by the plasma treatment. A new plasma rig was designed for this project, including a closed plasma chamber, two different parallel electrode configuration (flat or sawtooth electrodes) and a reel-to-reel system for the transport of the polypropylene foil. The designed system allows to adjust the sizes of the gas gap and the thickness of the dielectric. In addition, the plasma chamber can be filled with every gas or gas mixture wanted. A D-optimal design of experiments approach was utilised to study the influence of the power applied to the electrodes, the size of the gas gap and the thickness of the dielectric on the wettability of the treated polymer films. Furthermore, a flat electrode and a sawtooth electrode configuration are compared to each other. The findings of this work clearly indicate that the sawtooth electrode configuration is superior regarding the consumed electrical power and the wettability of the treated films. The optimal setting for the plasma treatment with the sawtooth electrode configuration are a low gas gap and a high power applied to the electrodes. The thickness of the dielectric has no influence on the wettability of the treated film. However, the increase of the dielectric thickness leads to an increase of the consumed power. These findings were transferred to an industrial process, where the polymer foils were treated in a nitrogen atmosphere with admixtures of CO2, N2O and C2H2 in the ppm range under the founded optimal conditions. A significant difference in the surface energies of these samples could not be identified by contact angles measurement, but all samples showed a significantly reduced hydrophobic recovery rate in comparison to samples treated by the in the industry more conventional plasma treatment in air. The DBD surface treatment in nitrogen is therefore an important improvement for the industrial production, because the treated packaging foils can be stored longer before they are processed further without the need to refresh the surface treatment. The low hydrophobic recovery of the samples treated in the nitrogen plasma is connected to an intensified crosslinking of the polymer chains on the surface of the treated polypropylene films.