Microhole Formation Behavior of Polypropylene Film Using CO₂ Laser Irradiation

Biaxially oriented polypropylene films produced by simultaneous equi-biaxial stretching to the draw ratio of 4×4 were perforated by the carbon dioxide laser under various laser fluences up to 426.3 J/cm2. After applying various laser fluences, the irradiated area was observed using a laser scanning confocal microscope. With the increase of fluence, the surface deformation with the formation of a crater surrounded by a doughnutlike rim was firstly observed at the laser fluence of 49.7 J/cm2, followed by the starting of the formation of a microhole surrounded by the rim at 113.7 J/cm2. The diameter of the microhole and the size of the rim increased rapidly with the increase of laser fluence. Mechanisms for the formation of a crater, the opening of a microhole, and the formation of surrounding rim were considered to be related to the surface tension and the shrinkage caused by molecular orientation relaxation. On the other hand, volume loss from the laser irradiated area was negligibly small at the moment of microhole opening, and started to increase rapidly when the fluence reached around 150 J/cm2. The microhole formation behavior was also observed by a high-speed camera. The velocity of microhole opening increased rapidly at first, presumably during the laser irradiation,and then slowed down after the cessation of laser irradiation. Theoretically estimated temperature distribution along the film thickness was compared with the laser perforation behavior. Starting of the deformation of film matched well with the starting of melting of the film surface. Film temperature was lower than the thermal decomposition temperature at the moment of the opening of a microhole, whereas at the moment of the starting of volume loss, film temperature was lower than the decomposition temperature estimated based on the heating rate of laser irradiated area, indicating that the decomposition and vaporization continues for a while after the cessation of laser irradiation.