## 1 Introduction
A typical arrangement of an industrial DBD system for polymer film treatment is illustrated in Figure 1. The aim here was to replicate the industrial system by developing a laboratory-scale DBD system with a reel-to-reel transport mechanism for the web and an electrode configuration consisting of one flat electrode covered with a dielectric and one electrode with a corrugated or sawtooth profile. This approach, published here for the first time, is designed to offer valuable insights into the surface treatment of polypropylene in an economic and rapid manner. DBDs at atmospheric pressure are classified into inhomogeneous filamentary discharges [21] and homogeneous or diffuse forms [22]. The type of discharge is dependent on operating parameters such as the amplitude, frequency and waveform of the voltage applied, the capacitance and surface properties of the dielectric material and of the uncovered electrode, the gap distance between the  
Figure 1: Schematic of industrial dielectric barrier discharges (DBD) configuration for the surface treatment of polymer webs. electrodes, and the composition of the gas in the gap. The discharge regimes are established through different breakdown mechanisms. The filamentary discharge is the most common form for industrial surface treatment of polymer webs at atmospheric pressure. The ignition of this discharge does not need special requirements and is thus easily established. If the applied AC voltage exceeds the breakdown voltage, the discharge occurs in the form of several thin single filaments, also called micro-discharges or streamers, in the gas gap between the two electrodes. The filaments are spatially and temporally separated from each other [6; 23]. Biaxially-oriented polypropylene (BOPP) films are most widely used in packaging applications throughout the world. Worldwide demand for BOPP films is estimated at almost 2.5 million tonnes, with Europe accounting for 33% of the global demand. Today, BOPP is the film of choice for packaging snack foods as well as non-food items such as computer diskettes, CD's etc. However, untreated BOPP has a low surface energy of about 30 mNm\({}^{-1}\). In order to maximise the adhesion between the BOPP and subsequent coating or lamination layers, the surface energy of the BOPP must be increased to a minimum of 42 mNm\({}^{-1}\)[2; 24]. The surface energy is also important for the wettability of the BOPP by printing inks. To achieve good printability, the BOPP needs a surface energy of at least 37 mNm\({}^{-1}\)[25]. The BOPP film utilised for this project (C50 grade, provided by Innovia Films Ltd., Wigton, Cumbria, UK) has a three-layer structure with two polyolefin heat sealable skin layers and a clear BOPP core layer. The heat sealable skin layers are made of a propylene-ethylene copolymer and have a thickness of 3 \(\upmu\)m, giving a total thickness of 50 \(\upmu\)m. One of the skin layers is corona treated, the other one is untreated. In all of the experiments described here, it was the previously untreated side that was plasma treated with the DBD and investigated. The objective is to treat the BOPP film in nitrogen and nitrogen containing other admixtures at atmospheric pressure with the aim of increasing the surface energy of the film.