Improvement in adhesion of polyethylene by glow-discharge plasma

The means by which plasma treatment enhances the adhesion of polymer materials, remains obscure. Thus far, two possible mechanisms have been proposed: an increase in surface energy, and the anchor effects imparted by plasma etching. Independently from these mechanisms, reactions between free radicals, generated by plasma irradiation and adhesives are also likely to affect the adhesive properties of polymer materials. Free radicals generated on polyethylene (PE) by glow-discharge plasma were exposed to air and converted to peroxide. The peroxides were converted back to free radicals with the application of heat, and then graft polymerization was initiated, by adding a hydrophilic monomer such as acrylic acid. The peroxides formed by the reaction between free radicals and the oxygen in air was detected by chemiluminescence (CL). In this work, plasma-treated PE surfaces were bonded to aluminum boards, using epoxy resin as an intermediate adhesive and then subjected to a series of peeling tests. The sample with the highest peeling strength also had the highest level of CL-detected peroxides. These findings suggest that the free radicals generated by plasma treatment influence the adhesive properties of the polymer materials.