Journal of Surface Engineered Materials and Advanced Technology, 2012, 2, 142-148 http://dx.doi.org/10.4236/jsemat.2012.23023 Published Online July 2012 (http://www.SciRP.org/journal/jsemat) Effect of Glow-Discharge Air Plasma Treatment on Wettability of Synthetic Polymers Ivo Krásný, Ivana Kupská, Lubomír Lapčík Centre of Polymer Systems, Faculty of Technology, Tomas Bata University, Zlin, Czech. Email: [email protected] Received April 16th, 2012; revised May 17th, 2012; accepted June 7th, 2012 ABSTRACT Main aim of this study was focused on characterization of the effect of microwave air plasma treatment on wettability of synthetic polymer surfaces. Wettability of solid polymer surfaces polyethylene, polypropylene, polystyrene (PE, PP, PS) was followed as a function of plasma treatment time. For evaluation the equilibrium contact angles of wetting as well as dynamic contact angles of wetting were determined by means of sessile drop and Wilhelmy plate methods. Free surface energy (SFE) of studied samples were calculated from the experimentally determined contact angles using Fowkes and van Oss, Chaudhury and Good (vOCG) approaches. It was found that with prolonged treatment time the total surface free energy of PE was two times increased from 23 mJ/m2 to 45 mJ/m2 after 360 s plasma treatment time (calculated for W and EG as wetting liquids). Similar effect was found for all studied polymers. With respect to the dispersive and polar components of the total surface free energy the vigorous effect was found for polar component, for which it was increased from 7 mJ/m2 to 20 mJ/m2. Keywords: Surface Energy; Contact Angle; Wetting; Plasma Treatment 1. Introduction measured by indirect methods—taking advantage of contact angle measurements. There are several well- The interfacial properties of polymer materials are im- known techniques of contact angle measurements on flat portant in many technological processes such as polymer and smooth surfaces e.g. sessile drop, adhering gas bub- blending, wetting, coating, and the reinforcement of poly- ble method and Wilhelmy plate method. The choice of mers with fibres. These are as a result of the complex particular method depends on geometry of the studied synergistic effect of surface morphology, topology and solid sample (i.e. flat plate, fibre, powder) [5-7]. chemical composition and homogeneity. The latter pa- Although, it is a difficult task to measure the contact rameters are affected both by nano-, micro- and macro- angle properly on solids, a large body of reliable data has scopic structural hierarchy of the surface layer arrange- been accumulated and a vast literature exist correlating ment. The wetting phenomena can be accessed by con- contact angle data with surface tension of solids [8]. tact angle measurements, which provide a better under- The aim of this work is to detect an influence of standing of the interactions between solids and liquids, or plasma treatment on wettability of synthetic polymers. between immiscible liquids [1,2]. Surface tension, or As an additional goal it is the verification of the accuracy surface free energy is a direct manifestation of acting of the free surface energy calculation according to intermolecular forces. The surface tension on the inter- Fowkes and vOCG. The latter materials are used as a faces is a very important parameter in basic physical basic support for attachment of molecularly oriented bio- chemistry of surfaces and in many areas of applied sci- polymers, thus their proper nanometric properties and ence and technology. That is why the surface modifica- characteristics are of paramount importance. tion plays a significant role [1,3]. Plasma treatment is an essential industrial process of surface modification. It is 2. Theoretical Background used to improve printability, wettability, bondability, biocompatibility, surface hardness, and surface heat re- A wide range of technological processes (e.g. cleaning, sistance etc [4]. dyeing or gluing) depends on “how well” the liquid wets The surface of a solid material possesses additional the surface of solid material. The term wetting and non- free energy, but owing to the reduced molecular mobility wetting as employed in various practical situations tend this free energy is not directly observable, and it must be to be defined in terms of the effect desired. Usually, Copyright © 2012 SciRes. JSEMAT Effect of Glow-Discharge Air Plasma Treatment on Wettability of Synthetic Polymers 143 however, wetting means that the contact angle between a Chaudhury and Good (vOCG). According to this method, liquid and a solid is zero or so close to zero that the liq- the surface tension of solid is equal to a sum of two LW uid spreads over the solid easily, and non-wetting means components: S , connected with Lifthitz-van der AB that the angle is greater than 90˚, so that liquid tends to Waals (LW) interactions, and S , connected with acid- ball up and run off the surface easily [5]. base (AB) interactions. The dispersion (London), orienta- Contact angle of wetting is a result of the balance of tion (Keeson), and induction (Debye) forces, being dipo- three vectors acting on the three-phase line among solid, lar forces, present the same type dependence on distance liquid and vapour, namely solid surface energy SV , and are usually treated with the same combining rules. liquid surface energy SV , and solid-liquid tension SL . All these interactions are generally named by the van der This exhibits an equilibrium contact angle that can be Waals contribution (LW). However, the donor-acceptor expressed by Young’s equation [9,10]: contribution to the work of adhesion (currently termed cos (1) acid-base contribution) is associated with the transfer of SG SL LG electrons between an electron donor and an electron ac- where SG and LG are the surface tension of a solid ceptor. and the surface tension of a liquid in the equilibrium with LWAB SS S (7) the liquid vapour, respectively, SL interfacial tension between the liquid and solid, and is the equilibrium AB 2SS (8) contact angle. It is to be pointed out that SG is not the surface tension of the solid ( in vacuum); the differ- S where S and S are components corresponding to ence in between is referred as the spreading pressure e : the interactions of the Lewis acid (donor) and base (ac- ceptor), respectively. Hence, the calculation of con- πeSS G (2) S sists in the determination of three unknown components: An assumption was made to neglect the vapour ad- LW S , S and S , it is required performing contact sorption for low surface tension solids, e.g. polymers, angle measurements using different liquids, two of them SS G [11,12]. being polar and one non-polar. Combining Equations (7) According to the Fowkes, the SFE is separated into its and (8) with Young’s equation is given: components: (1 cos ) 2 LW LW (9) CIDABH (3) llslsl s where D ,,PHI , and AB are the contributions The vOCG approach to the evaluation of acid-base from London disperse forces, polar (Keesom forces, hy- properties of solid surfaces by contact measurements is drogen-bonding forces, induction (Debye) forces, and undoubtedly the most important development of pre- acid-base interactions, respectively. Based on these as- sent-day interface science [13-16,17]. sumptions, the following equation has been proposed, which is applicable to systems in which only dispersion 3. Experimental forces are common to both phases: 3.1. Materials D D 12 1 22 1 2 (4) As a polyolefin sheets (120 × 120 × 2 mm) tested in this where subscripts 1 and 2 represent phases 1 and 2, re- study has been prepared: Polyethylene (PE-LDPE Bralen spectively. Applying Equation (4) to a solid-liquid inter- RA 2-63), 5 min at temperature of 180˚C, polypropylene face in given: (PP-Chemopetrol N 159-93), 5 min at temperature of 227˚C and polystyrene (PS-Krasten 154), 5 min at tem- D D SL S L2 S L (5) perature of 200˚C by injection molding machine. If π is assumed to be small and , can e LVLS 3.2. Methods be eliminated by combining Equation (5) and Equation (1): Plasma surface treatment was performed in reactor built by Vakuum Praha at the Institute of Physics and Material D D cos 1 2 SV LV (6) Engineering, Thomas Bata in Zlin. Radio-frequency dis- LV charge was excited at 13.56 MHz with inner diameter of D 80 mm equipped with plain parallel electrodes placed in –3 Then the value of SV can be calculated from the the 50 mm distance. Operating pressure was 0.5 × 10 D slop of cos plotted as a function of LVLV Ba. Air was used as a discharge gas. Apparatus was con- [7,13,14]. sisted of rotary and diffusion vacuum pump, vacuum The most recent approach was given by van Oss, meter TPG 215 (Pfeiffer), mass flow controller (Cole Copyright © 2012 SciRes. JSEMAT 144 Effect of Glow-Discharge Air Plasma Treatment on Wettability of Synthetic Polymers Palmer). The samples of PP, PE, PS were exposed in the and a tested solid interface (synthetic polymer) as shown following time range 0 - 360 s. The time between exposi- in Figure 1 and data given in Tables 2 and 3. tion and experimental measurement was not longer than 30 min. Table 1. Surface tensions and components (mJ·m–2) sug- The magnitude of the contact angle, , between the gested by vOCG [16]. liquid and solid can be determined by various methods. LW The selection of the method to be used depends on the Wetting liquid LV LV LV LV character of the studied sample system, as well as on the accuracy required. The two most common methods are: a Deionised distilled water 72.8 25.5 25.5 21.8 static measurement of sessile drop by See System with CCD camera and a dynamic Wilhelmy plate method.