Analysis of White Stripe Phenomenon of Polypropylene Thin-walled Injection Molded Products
In this paper,to reveal the causes of white strip phenomenon of polypropylene(PP)thin-walled injection molded products,homopolymer polypropylene(PP-A and PP-B)were analyzed in terms of molecular weight distribution,chemical composition distribution,oxidation induction time,and crystallization behaviors.During the thin-walled injection molding process,PP-A formed white strips marked at the bottom of the molded specimen.The white marks radiated from the center outwards,deteriorating the appearances of the products.The effects of molecular chain structures and thermal oxidation were excluded by characterizations.The white marks disappeared after an isothermal treatment process at 165℃for 5 min in an oven.Crystallization behaviors and crystal morphology characterizations show that,inside the area of the white marks of PP-A,the crystal morphology is not uniform,with a banded distribution of large crystal grains.In the transparent areas of PP-A and PP-B,the crystal grains are evenly distributed with small sizes.It is concluded that the differences in the appearances of different regions of PP-A products are caused by the differences in crystal morphologies,likely related to nucleation agents.This paper provides a systematic analysis scheme for exploring the causes of white stripes in polypropylene thin-walled injection molded products,and references for the applications and product upgrading of thin-walled injection plastics.