FLAT-FILM PROCESSING IN ELECTROSTATIC FIELD

Abstract

A high density polyethylene was subjected to the flat-film casting through a 20cm coathanger die coupled to a 25mm single screw extruder. A specially designed electrode, consisting of 26 needles vertically aligned on a surface of a thin aluminum strip at intervals of 2mm, was placed several centimeters apart under the coathanger die and connected to a high voltage direct current generator, while the chill-roller of the film processor being grounded so as to act as the counterelectrode. The molten running film emerged through the die at the linear velocity of 0.75m/min. was drawn down in the electrostatic field generated between these electrodes and immediately frozen on the chill-roller rotating at the surface velocity of 15m/min. The variation of the film width was measured as a function of the applied voltage and the film-to-electrode distance. When the applied voltage exceeded 4kV, the running film began to spread in width and to retract in thickness. The width increased drastically at the voltages ranging from 6kV to 18kV. Above 20kV it gradually levelled off to the value 1.4 times as wide as that at 0kV. On the other hand, the film width was almost independent of the film-to-electrode distance. The mechanism for spreading width was considered as follows: The corona shower from the electrode put the sufficient quantity of the electric charge on one side of the film surface and subsequently induced the corresponding charge of opposite sign on the surface of the chill-roller contacting to the other side of the film. The coulombic attractive force between these positive and negative charges might press the film in the direction of the film thickness and it consequently resulted in the spreading of the film in both longitudinal and transverse directions. The longitudinal contribution, however, probably reduced the draw-down tension because of the large draw-down ratio, whereas the transverse one might make the neck-down smaller. Finally it must be noted that the film thus prepared was very transparent and had a very clear surface.