## 2 Experimental Section
### Treatment of BOPP Film in Nitrogen  
An experimental array was developed to investigate the influence of specific process parameters during the treatment of the BOPP film in a nitrogen atmosphere and to identify the optimum operating conditions for the DBD reactor. The following three factors were chosen for the experimental array:  
* the thickness of the dielectric covering the upper electrode (four levels were tested: 0.63 mm, 1.26 mm, 1.89 mm, 2.52 mm)
* the size of the gas gap measured between the upper electrode and the top of the teeth on the sawtooth electrode (0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm)
* the DC power supply current setting (0.8 A, 1.0 A, 1.2 A, 1.4 A)  
These factors and values represent the practical extent of the process envelope for this system. The optimal frequency of the function generator for this experimental configuration was found to be between 29.6 and 29.9 kHz. All other factors were kept constant throughout the array. Based on the D-optimal approach, the experimental array that was developed from these parameters is listed in Table 1. Each experiment was repeated three times.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number of** & **Current [A]** & **Thickness of the** & **Size of the Gas** & **Optimal** \\
**Experiment** & **C**urcent & **Dielectric [mm]** & **Gap [mm]** & **Frequency [kHz]** \\ \hline
1 & 0.8 & 0.63 & 2.0 & 29.8 \\
2 & 1.4 & 0.63 & 0.5 & 29.6 \\
3 & 1.2 & 1.26 & 0.5 & 29.6 \\
4 & 1.2 & 0.63 & 2.0 & 29.8 \\
5 & 0.8 & 0.63  
The experimental procedure was as follows. A 30-cm-long, 3-cm-wide BOPP sample was fixed to the reel-to-reel system with adhesive tape. The untreated side of the BOPP film faced the gas gap and the whole film was coiled on the unwind reel. The reel-to-reel system was then placed on its bracket inside of the plasma chamber, which was then sealed with the end cap. The rotary pump was used to evacuate the chamber to a minimum pressure of 1300 Pa. When this pressure was reached, the nitrogen gas valve was opened. The rotary pump was allowed to run for a further 2 min to flush the chamber with the process gas. The pressure inside the chamber was set to a slight overpressure of 1.05 bar to prevent the surrounding air entering the chamber.  
After the plasma treatment, the process chamber was opened and the BOPP film removed from the reel-to-reel system. Ten drops of distilled water were placed along the centre line of the treated surface of the BOPP film at a separation of approximately 1 cm from each other and the contact angles of these drops were measured. Contact angle measurements were made at room temperature using the sessile drop technique and 5-\(\upmu\)L volumes of solution on a Kruss goniometer (Hamburg, Germany) and data analysis system [30]. The average of the ten contact angles was taken as the response of the experimental array. The first distilled water drop was placed 7 cm away from the end of the sample that had been attached to the rewind, as this section of film did not pass between the electrodes.  
Mean values of contact angle were taken for each experiment and from these values, level averages were calculated for each level of each factor. These level averages indicated the relative influence of each factor on the water contact angle and the direction of any trend with the variable. Based on these findings, operating conditions were selected to minimise the water contact angle for subsequent experiments.