Effect of molding history on molecular orientation relaxation during physical aging of polystyrene injection moldings

Kousaku Tao; Koji Yamada; Seiji Higashi; Keitaro Kago; Shiho Kuwashiro; Hiroshi Hirano; Hiroki Takeshita; Katsuhisa Tokumitsu

doi:10.1515/ipp-2022-4264

Article

Effect of molding history on molecular orientation relaxation during physical aging of polystyrene injection moldings

Kousaku Tao , Koji Yamada , Seiji Higashi , Keitaro Kago , Shiho Kuwashiro , Hiroshi Hirano , Hiroki Takeshita and Katsuhisa Tokumitsu

Published/Copyright: February 17, 2023

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From the journal International Polymer Processing Volume 38 Issue 2

Abstract

This work examined the effect of changing molding conditions on the physical aging of polystyrene injection moldings. First, we investigated the relationship between the molecular orientation and the molding conditions. The molecular orientation near the surface changed with changing injection rate, so we hypothesized that this molecular orientation might form during the filling stage. Because this molecular orientation did not relax under heat treatment below the glass transition temperature (T_g), the oriented molecules near the surface were thought to be elongated owing to the high strain rate during the filling stage. On the other hand, the molecular orientation in the core layer changed with changing holding pressure and relaxed under heat treatment below T_g. Thus, the molecules in the core layer might become oriented during the holding stage and not be elongated owing to the slow strain rate. Furthermore, the molecular orientation in the core layer decreased with increasing mold temperature, and the physical heat resistance improved with increasing mold temperature. Meanwhile, the excess enthalpy did not change with changing molding conditions. Therefore, the improvement in physical heat resistance with increasing mold temperature was likely caused by the decrease in the molecular orientation in the core layer. Analyzing the relaxation behavior of the molecular orientation suggested that increasing mold temperature reduced the number of oriented molecules with large deformation in the core layer.

Keywords: injection molding; molecular orientation; physical aging; polystyrene; relaxation

Corresponding author: Kousaku Tao, Polymer Processing Engineering Laboratory, Research Division of Materials Science and Engineering, Morinomiya Center, Osaka Research Institute of Industrial Science and Technology, 1-6-50, Morinomiya, Joto-ku, Osaka-city, Osaka 536-8553, Japan; and Department of Material Science, School of Engineering, University of Shiga Prefecture, Hikone-city, Japan, E-mail: tao@orist.jp

Acknowledgements

Mark Kurban from Edanz (https://www.jp.edanz.com/ac) edited a draft of this paper.

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2022-08-10

Accepted: 2023-01-17

Published Online: 2023-02-17

Published in Print: 2023-05-25

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Articles in the same Issue

https://doi.org/10.1515/ipp-2022-4264

Keywords for this article

injection molding; molecular orientation; physical aging; polystyrene; relaxation