Synopsisit-Polypropylene films were coextruded by using dies of varying extrusion draw ratio (EDR) from 2-10. Among the resulting films, the film coextruded at EDR 9 possesses the most attractive superstructure and the highest modulus. The dynamic modulus at room temperature was 7 GPa and an a, dispersion peak occurred at 100°C in E"-temperature curves, which is higher by about 2OoC than that of the multistep zone-drawn and zone-annealed fibers previously reported. Further, to prepare films with higher draw ratio and higher modulus, repetition of the coextrusion was attempted. Consequently, the draw ratio and the E' at room temperature reached 12 and 13 GPa, respectively. The present paper discusses changes in molecular orientation, crystallinity, uniformity of the superstructure, and dynamic viscoelasticity, with increasing of the used EDR and repeating the coextrusion with some combinations of EDR.
SYNOPSISIn order to improve the mechanical properties of polypropylene film, a new processing combining extrusion and zone-annealing has been applied. It was found that there are suitable conditions for each step in the combined processing. When the coextrusion draw ratio was low, the total draw ratio and modulus could be increased by the zone-annealing subsequently done. The highest modulus was obtained when the film was coextruded at extrusion draw ratio 4 and then zone-annealed at 120°C under 7 kg/mm2. The value was 12 GPa in Young's modulus or 17 GPa in dynamic modulus. The peak temperature of a, dynamic dispersion for the combinedly processed film was 109"C, which is higher by 10°C than that for the as-coextruded film. Four drawing methods were compared in dynamic viscoelasticity. These methods are the coextrusion, zone-drawing/zone-annealing, twostep coextrusion, and the combined processing by coextrusion and zone-annealing. The highest dynamic modulus for each method was arranged in the above order. The combined processing indicated the most effective improvement in mechanical properties, because it is believed that lamellae in the original film were broken by cooperating interaction of shear stress, compression, and tension on coextrusion and then the superstructure with a high crystallinity and a high molecular orientation was formed on zone-annealing.
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