Low-Energy Ar + Implantation of Uhmw-Pe Fibers: Effect on Surface Energy, Chemistry, and Adhesion Characteristics
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LOW-ENERGY AR+ IMPLANTATION OF UHMW-PE FIBERS: EFFECT ON SURFACE ENERGY, CHEMISTRY, AND ADHESION
CHARACTERISTICS
R. SCHALEK, M. HLAVACEK and D. S. GRUMMON
Department of Metallurgy, Mechanics and Materials Science, Michigan State University, East Lansing, MI 48824. ABSTRACT
Ultra-high molecular weight polyethylene (UHMW-PE) has a highly chain-extended and crystalline structure which is functionally inert and requires surface-modification before it can successfully operate as a reinforcement in polymer-matrix composites. Although plasma treatments are adequate for this purpose, recent work has shown that irradiation with low-energy inert gas ions can produce increases in interfacial shear strength (ISS), in epoxy matrices, which exceed those of commercial plasma treatments, and cause little degradation in tensile properties. Low energy ions are readily produced in high-current beams using gridded sources having moderate cost, and processing times may be a short as a few seconds. In this paper, we present results of recent experiments using argon ions accelerated to energies between 100 eV and 1 keV to irradiate 20-30 gim diameter UHMW-PE fibers to doses between lx1016 and lx10 17 cm- 2, and compare our findings with previous work at higher accelerating potentials. At the optimum dose (which increases with decreasing energy), greater than 9-fold improvements in ISS level, measured in epoxy-resin droplet pulloff tests, were found for ion irradiation at 0.25 keV. Scanning electron microscopy of fiber surfaces, of ion irradiated as well as commercial oxygen plasmatreated materials, revealed small crack-like pits in both cases, with the pits smaller and more uniformly distributed on the ion-irradiated fibers. Surface chemistry studies using X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) indicate that irradiation resulted in high surface concentrations of polar functional groups, and extensive surface oxidation. This was accompanied by a substantial increase in the polar component of surface energy, which resulted in improved fiber wetting by the resin. INTRODUCTION
Adhesive bonding of polyethylene to other materials is generally difficult, but can be substantially facilitated if the surface is preprocessed by means such as electrical discharge or chemical treatment. For the case of ultra-high molecular weight polyethylene (UHMW-PE) fibers, successful use in advanced polymer-matrix composites requires good adhesion in order to provide adequate matrix-fiber load transfer. Commercial surface modification processes for polyethylene fibers are designed to control the chemical surface functionality, surface topography and/or to remove weak boundary layers containing low molecular weight oligomers or other additives, with the general objective of improving adhesion, wettability and structural integrity of the fiber surface region. Though many fiber surface treatments have met with success, the techniques are highly parameter-sensitive and often nega
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