Surface modification of PP under different electrodes of DC glow discharge and its physicochemical characteristics S. Bhowmik a , P. Jana a , T.K. Chaki a, * , S. Ray b a Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India b Department of Metallurgy and Materials Engineering, Indian Institute of Technology, Roorkee 247667, India Received 3 June 2003; accepted in revised form 19 December 2003 Available online Abstract The present investigation aims to study the wetting and physicochemical characteristics of Polypropylene (PP) sheet when exposed to a DC glow discharge through air across different electrodes such as copper, nickel and stainless steel at 13 W power level of DC glow discharge under an electrode size of 64 cm 2 . In order to estimate the extent of surface modification, the surface energies of the polymer surfaces exposed to DC glow discharge have been determined by measuring contact angles using two standard test liquids of known surface energies. It is observed that surface energy and its polar component increases with increasing exposure time, attains maximum and then decreases. The increase in surface energy and its polar component is relatively higher when the polymer is exposed under stainless steel electrode followed by nickel and then copper electrode. The dispersion component of surface energy remains almost unaffected. The surfaces have also been studied by optical microscopy and electron spectroscopy for chemical analysis (ESCA). It is observed that when the PP is exposed under these electrodes, single crystals of shish kebab structure forms and the extent of formation of crystals are higher under stainless steel electrode followed by nickel and then copper electrodes. Exposure of the polymer under DC glow discharges has essentially incorporated oxygen functionalities on the polymer surface as detected by ESCA. It is noted that higher incorporation of oxygen functionalities have been obtained, when the polymer is exposed to DC glow discharge under stainless steel electrode followed by nickel and copper electrodes as evident from ESCA studies. These oxygen functionalities have been transformed into various polar functional groups and which has been attributed to increase the polar component of surface energy of the polymer. Therefore, maximum increase in surface energy is obtained when the PP sheet is exposed under a stainless steel electrode rather than nickel or copper electrodes. D 2004 Published by Elsevier B.V. Keywords: Polypropylene; DC glow discharge; Electrode; Surface energy; Crystallisation; Oxygen/carbon ratio 1. Introduction Polymeric materials like polypropylene (PP), polyethyl- ene (PE), polycarbonate (PC) etc. are progressively replacing the traditional engineering materials like steel and aluminium in fabrication of secondary structures of aircraft, automo- biles, railway coaches, civil construction as well as biomed- ical application due to their superior properties like better corrosion resistance, high strength to weight ratio, relatively low cost and easy recycling [1,2]. However, unfortunately, surfaces of these polymers are hydrophobic in nature due to absence of polar functional groups and exhibit low surface energy and therefore, show technological challenges in the field of printability, biocompatibility and especially, adhe- sion of polymeric coating as well as adhesion of polymer surface and even polymer to other materials. Hence, surface modification of polymer is often carried out to make polymer surface hydrophilic by incorporating polar functional groups leading to increasing surface energy. Several surface modification methods are employed to modify the polymer surfaces, such as chemical treatments, thermal treatment, mechanical treatment and electrical treat- ments under: (a) atmospheric pressure plasma (corona dis- charge); and (b) low pressure plasma (glow discharge). Glow discharge under low pressure plasma is a popular technique, which results in better uniformity in surface modification of the polymers [3–6]. Moreover, it is a dry treatment method, which is better suited for industrial applications. It is now 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2003.12.013 * Corresponding author. Tel.: +91-3222-283182; fax: +91-3222- 255303. E-mail address: tapan@rtc.iitkgp.ernet.in (T.K. Chaki). www.elsevier.com/locate/surfcoat Surface & Coatings Technology 185 (2004) 81– 91