Corona‐discharge treatment of polymeric films, II: Chemical studies

Polymeric films, chiefly polyethylenes, were subjected to corona‐discharge treatment in a continuous treater at commercial rates in a program covering wide ranges of the main processing factors (2). Electron‐spin‐resonance measurements on freshly treated films found no free radicals. Reactions of the treated surfaces with a free‐radical compound, diphenyl picryl hydrazyl (DPPH) were studied, focusing mainly on the rate effects. The evidence indicates that corona treatment produces fairly stable peroxide structures of the forms RO 2 R and RO 3 R on polyethylene surfaces. RO 3 R reacts rapidly with DPPH alone, while RO 2 R undergoes a slower reaction after addition of the catalyst, triethylene diamine. DPPH is capable of detecting as few as 10 13 peroxide groups per square centimeter. Activation energies were 12 kcal/mole for the uncatalyzed reaction and 16 kcal/mole for the amine‐catalyzed reaction. As with the physical effects reported earlier (2), the production of peroxides is most strongly dependent on the energy delivered to the film during treatment. This energy is proportional to the quotient of corona current and web speed, I/S, Regression analysis showed that air‐gap thickness, relative humidity, and number of electrodes used also were significant factors, while dielectric thickness and corona frequency were not. We found that‐γ , the polar component of surface energy of the treated film, which is nearly zero for untreated polyethylenes, is exponentially related to the concentrations of both RO 2 R and RO 3 R with a correlation coefficient for 92 specimens tested of 0.88. We believe this is the first strong evidence linking treatment factors, at commercial levels of treatment, to chemistry of the treated surface and linking both of those to changes in physical behavior of the surface.