Effective surface charge densities on two surfaces of polystyrene films charged by friction and corona discharge and their polarity

Abstract

Effective surface charge densities (σ ∗) were measured for both surfaces of polystyrene (PS) films charged by friction with nylon 6 (Ny) films and corona discharge. In the films charged by friction, the polarity of σ ∗ on charged (S1) and uncharged (S2) surfaces was negative. The absolute value of σ ∗ on S1 was larger than that on S2. After rinsing the charged films with ethyl alcohol, S1 carried negative charge but S2 positive charge. The series of frictional electrification predicts the polarity of charge on PS rubbed by Ny before rinsing. However, it should be noted that the films were polarized distinctly with a small net charge. In temperature dependence of σ ∗, a peak was found at about 40°C and an increase of σ ∗ was noticed in the higher temperature region up to 100°C. They were explained by thermal motion of PS molecules. In corona electrification, the polarity of σ ∗ on both surfaces changed when the distance between the film and an earthed electrode was varied: when negative corona was emitted to the film put directly on the earthed electrode S1 carried negative charge but S2 positive charge. When the film was suspended 3.7 mm above the electrode, S1 and S2 carried negative charges. Charge decay was measured for the films charged by friction and corona discharge and analyzed using a three-exponential-sum equation to obtain relaxation times. Two mechanisms of charge decay were considered: one caused by surface charges and the other by charges injected into surface layer. Thermally stimulated depolarization current was also investigated. All the data showed that the behaviour of charges on S1 was different from that on S2.