Atmospheric-pressure plasma treatment of ultrahigh molecular weight polyethylene fibers

Abstract

The performance properties of composite materials reinforced by ultrahigh molecular weight polyethylene (UHMW-PE) fibres depend critically on the fiber/matrix interfacial characteristics. The chemical inertness and very low surface energy of UHMWPE fibers, however, results in a fiber/matrix interfacial strength much less than ideal for such applications [1]. Several teams made remarkable achievements in the surface modification of UHMW-PE by low-pressure plasma treatment at pressures below one Torr (130 Pa) [2–6]. However, these low-pressure plasma treatments require vacuum systems and do not lend themselves easily to an industrial implementation. In this study UHMW-PE fibers were treated by a nitrogen plasma at atmospheric pressure, where the plasma treatment can potentially be realised with less technical effort. A nitrogen plasma was chosen because of its low degrading effect and high radical density creation [6, 7]. A commercially available UHMW-PE filament bundle of 150 filaments (Dyneema SKX 65, dtex 165) was cleaned with acetone before the treatment. The plasma treatment was performed using a pulsed surface discharge reactor that consisted of two on-axis-arranged electrodes housed in a glass chamber [8]. Nitrogen of a technical purity was introduced into the chamber with a flow rate of 1 cm3 s−1. The ground stainless-steel tubular anode was 1 mm in inner diameter. The cathode was a 15 mm-diam. hemispherically capped brass rod with a 2 mm-diam. hole in its axis. The distance between electrodes was adjusted to 15 mm. The treated filament bundle moved on the axis of the electrode system with a speed of 100 mm s−1. The cathode was connected with a tyratron source of pulsed high voltage with a pulse frequency of 100 Hz, a peak voltage of 25 kV, pulse rise time of 75 ns, and pulse half-width of 400 ns. The discharge power was approximately 10 W. To illustrate the effect of the plasma treatment on fiber surface properties, the fiber/rubber matrix adhesion values measured using the untreated and treated fibers were compared. The samples were prepared by pressing the filament bundle between two slides from a conventional polybutadiene rubber blend. Subsequently, the samples were vulcanised at a temperature of 145 ◦C. Adhesion of the fibers to rubber was characterised in a standard H-peel test by the force required to draw out the filament bundle of a rubber block. The adhesion values (the averages of twenty measurements) for the reference and plasma-treated fibers were 1.7 and 2.5 kN m−1, respectively, i.e. an increase of 44% was obtained by the plasma treatment. As stated by Jacobasch et al. [9], some insight into the adhesion behavior of polymer surfaces can be obtained also by zeta potential measurements with varied electrolyte concentration. Preliminary results of our ζ -potential measurements are shown in Fig. 1. From the results it is evident that anions (Cl− and OH−) present in KCl electrolyte are preferably adsorbed by the treated fiber surface. The observed decrease in negative ζ -potential values due to the plasma treatment (corresponding to a decrease of the electrical double layer thickness [9]) indicates a higher hydrophilicity of the plasma-treated fibers. An increase in the pH corresponding to the zero ζ -potential (isoelectric point) indicates a creation of basic surface groups by the plasma-treatment. This is consistent with the basicity of polyethylene tapes treated by atmospheric pressure nitrogen plasma [10] found on the base of contact angle measurements. Our ζ -potential measurements, in an agreement with the results of Gresseer et al. [11] and Wakida et al. [12], affirm that ζ -potential measurements can be a useful technique in determining of technologically