EFFECT OF SURFACE MOISTURE ON CONTACT CHARGE OF POLYMERS CONTAINING IONS

The effect of humidity on the contact charge which develops between th e ion-containing polymeric powders and steel beads was examined. The p owders are a blend of a styrene-butyl methacrylate copolymer and minor amounts of the acid form of a partially sulfonated polystyrene ionome r or the corresponding sodium salt. At 0% relative humidity (RH), the contact charge on the blends is effectively that of the host copolymer . It rises as the humidity increases from 0 to 20-40% RH and then decr eases at the higher humidities. A higher contact charge is attained wi th the acid form of the ionomer than with the corresponding sodium sal t which in turn charges higher than the copolymer. The surface water l ayer thickness of the blends was measured by ellipsometry and also fou nd to increase with humidity. The optimal charge is observed in the hu midity range where about one monolayer of water adsorbs on the polymer and oxide surfaces. The decrease in charge at the higher humidities i s attributed to the ineffective mixing between the powder and the bead s during rolling, plus charge ''run-off'' due to reduction of the surf ace electrical resistance as detected by ac dissipation. The contact c harge is attributed to the mobilization of dissociated ions (from the ionomer) in the surface water layer followed by ion transfer to the se cond surface during the contact.