The Morphological Evolution Of Polypropylene In The Stretching Field

Isotactic polypropylene (iPP) resin has been widely applied in production and processing, because of the excellent performance. However there are some defects in PP, such as poor impact strength and low heated deformation temperature, etc. which have limited the application in some field. In order to improve the performance of PP, it can induce the formation of theβ-form by many methods. It has been reported thatβ-crystal exhibits many excellent characteristics such as improved elongation at break and impact strength, the transparency and flexural strength of PP can significantly be mended as well, because of this, it has drawn much interest.In this work, the pure iPP and that containing different mass percent ofβ-nucleating agent (β-NA) were extruded through a slit die (Haake Polylab System-Rheomex 252p series). Simultaneously, the extruded melt was stretched with different stretching rates (SR). In order to diminish the shear stress in the die as much as possible, the rotating speed of screw was small enough. For P-crystal can grow in special crystallization temperature range, we are preparing an isothermal box, by which the stretched sheets can be crystallized in the same isothermal conditions, such as 100℃and 120℃. PLM, WAXD and DSC measurement were employed to study the effect of SR, crystallization temperature andβ-NA on the occurrence ofβ-crystal, the orientation and the melting transition. A few conclusions can be summarized as follows:(1) The influence of factors on the orientation of iPP:The orientation degree increases with the increasing SR. The orientation of the sheets cooled in the air is higher than that of sheets crystallized in 100℃and 120℃.(2) The influence of factors on the generation ofβ-crystal:Littleβform exited in pure iPP. The growth ofβ-crystal was promoted at lower SR and suppressed at higher SR. However, for all theβ-nucleated iPP, it decreases monotonously with the increasing SR. The samples crystallized in high temperature are favor to produceβmodification, theβ-crystal content of which is higher than that of sheets cooled in air.(3) The effect of heat treatment on the multiple melting behavior of iPP: Multiple melting peaks appeared in DSC curves of the sheets stretched at different, which are attribute to the fusion of the crystal with different orders of perfection, thermal stability or lamellar thickness.