Single and Double Bubble Tubular Film Extrusion of Polyethylene Terephthalate

Abstract

Abstract An experimental study of double bubble tubular film extrusion of polyethylene terephthalate is presented. In the second bubble, the film is passed through a hot air where it undergoes a significant increase in inflation level. The mechanical stability of both bubbles is investigated as a function of process conditions. The film produced by the first bubble is largely amorphous/glassy (5 to 7%, crystallinity), while that produced in the second bubble can have significant levels of crystallinity (7 to 45 %) dependent upon process conditions. Crystallinity notably increases with machine direction draw down ratio. The crystallinity levels are similar to those of biaxially stretched films. Subsequent annealing increases and perfects crystallinity. The surface roughness of double bubble tubular films is similar to that of glassy polymers produced from single bubble operations and is much less than for polyolefins. The orientation in the films has been determined by (i) wide angle x-ray diffraction applying both pole figure analysis and biaxial orientation factors (ii) measurement of in-plane and out-of-plane birefringence. Biaxiality of orientation increases with the second blowup ratio. The mechanical properties of the film were determined as a function of angle in the plane of the film.