Adhesive properties of polypropylene (PP) and polyethylene terephthalate (PET) film surfaces treated by DC glow discharge plasma K. Navaneetha Pandiyaraj a , V. Selvarajan a, * , R.R. Deshmukh b , Changyou Gao c a Plasma Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, India b Department of Applied Physics, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India c Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China article info Article history: Received 8 February 2008 Received in revised form 26 May 2008 Accepted 26 May 2008 Keywords: Polypropylene Polyethylene terephthalate Glow discharge plasma XPS AFM Adhesion abstract In this study, the adhesive properties of the plasma modified polypropylene (PP) and polyethylene terephthalate (PET) film surfaces have been investigated. Hydrophilicity of these polymer film surfaces was studied by contact angle measurements. The surface energy of the polymer films was calculated from contact angle data using Fowkes method. The chemical composition of the polymer films was analyzed by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) was used to study the changes in surface feature of the polymer surfaces due to plasma treatment. The adhesion strength of the plasma modified film was studied by T-peel strength test. The results showed a considerable im- provement in surface wettability even for short exposure times. The AFM and XPS analyses showed changes in surface topography and formation of polar groups on the plasma modified PP and PET sur- faces. These changes enhanced the adhesive properties of polymer film surfaces. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Polymeric materials like polypropylene (PP) and polyethylene terephthalate (PET), etc. are increasingly replacing the traditional engineering materials like steel and aluminum in fabrication of secondary structures of aircraft, automobiles, railway coaches, civil construction as well as biomedical application due to their superior properties like better corrosion resistance, high strength to weight ratio, relatively low cost and easy recycling [1–4]. However, poly- mers are innately hydrophobic, low surface energy materials, and thus do not adhere well to other materials. It is necessary to modify their surfaces to increase the surface energy without change in their bulk properties for many commercial applications. Several surface modification methods are employed to modify the polymer sur- faces, such as chemical, thermal, mechanical and electrical treat- ments [5,6]. Recently, research on the use of plasma treatments has grown in interest, since they are environment friendly [7–9]. The glow discharge plasma treatment is a popular technique since it is a dry process and allows better uniformity in the modified surface, and hence it is widely used for industrial applications. The plasma acting mechanisms are very complex, but the main results obtained are surface cleaning, activation, crosslinking, etching or, in most cases, combined effects [8]. The action of the plasma promotes the formation of free radicals that can act as interlock points for active species (polar groups) [10]. Furthermore, depending on the plasma forming gas and general conditions of the plasma treatment, it is possible to promote some surface etching/abrasion which can in- duce changes in surface topography, thus having a positive effect on the wettability improvement [11]. In this paper, polypropylene and polyethylene terephthalate film surfaces were modified through air plasma with an aim of improving its adhesive properties. 2. Experimental setup and methodology Low-pressure glow discharge plasma was generated in a plasma chamber made of glass of length 29 cm and internal diameter 10 cm (Fig. 1). The discharge chamber was first thoroughly cleaned. Initially the chamber was evacuated to a pressure of 10 3 mbar using a vac- uum pump (EDWARDS: E2M5). Through an air inlet (fine control gas needle valve) the required low pressure was maintained and mea- sured by a Pirani gauge. Electrodes were made of aluminum. They were circular in shape with a diameter of 5 cm. Electrodes were fixed inside the chamber axisymmetric and perpendicular to the axis and were separated by a distance of 3 cm. Plasma electron density and temperature were measured using a Langmuir probe. The probe is made of a tungsten wire (length ¼ 2 cm and diameter ¼ 0.05 cm) encapsulated in a glass sleeve. The plasma electron density and temperature were 9.2  10 13 cm 3 and 2.9 eV, respectively. The hydrophilicity of the PP and PET films was characterized by * Corresponding author. Tel.: þ91 422 242 6576/8130. E-mail address: vselvrjn47@rediffmail.com (V. Selvarajan). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.05.032 Vacuum 83 (2009) 332–339