Cracking Behavior Analysis of Thermoplastic Polyimide under Vacuum Pressure Impregnation

Chao Xiao; Xian Zhang; Long-Gui Zheng; Ye-Zheng Xiao; Yan-Lan Hu

doi:10.1007/s10118-026-3878-0

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Cracking Behavior Analysis of Thermoplastic Polyimide under Vacuum Pressure Impregnation

RESEARCH ARTICLE | 更新时间：2026-11-03

- Cracking Behavior Analysis of Thermoplastic Polyimide under Vacuum Pressure Impregnation
- Chinese Journal of Polymer Science Vol. 44, Pages: 1-12(2026)
- 作者机构：
  
  Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
- 作者简介：
  
  zlonggui@ipp.ac.cn
- 基金信息：
- DOI：10.1007/s10118-026-3878-0
  CLC：
- Received：09 June 2026，
  
  Accepted：07 September 2026，
  
  Online First：02 November 2026，
  
  Published：2026-10
- 稿件说明：
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Xiao, C.; Zhang, X.; Zheng, L. G.; Xiao, Y. Z.; Hu, Y. L. Cracking behavior analysis of thermoplastic polyimide under vacuum pressure impregnation. Chinese J. Polym. Sci. https://doi.org/10.1007/s10118-026-3878-0

Chao Xiao, Xian Zhang, Long-Gui Zheng, et al. Cracking Behavior Analysis of Thermoplastic Polyimide under Vacuum Pressure Impregnation[J/OL]. Chinese Journal of Polymer Science, 2026, 441-12.
Xiao, C.; Zhang, X.; Zheng, L. G.; Xiao, Y. Z.; Hu, Y. L. Cracking behavior analysis of thermoplastic polyimide under vacuum pressure impregnation. Chinese J. Polym. Sci. https://doi.org/10.1007/s10118-026-3878-0 DOI：

Chao Xiao, Xian Zhang, Long-Gui Zheng, et al. Cracking Behavior Analysis of Thermoplastic Polyimide under Vacuum Pressure Impregnation[J/OL]. Chinese Journal of Polymer Science, 2026, 441-12. DOI： 10.1007/s10118-026-3878-0.

摘要

Abstract

Thermoplastic polyimide (TPI) insulated wires

which exhibit excellent insulating and radiation-resistant properties at extremely low temperatures

are designated as connection lines for the quench detection system of superconducting magnets. However

extensive cracking was unexpectedly observed after the implementation of vacuum pressure impregnation (VPI). The cracking behavior of TPI-insulated wires during the VPI process was systematically investigated. The results reveal that the failure is a multi-factor coupled phenomenon involving synergistic interactions between thermal

mechanical

and chemical mechanisms. First

the coiled configuration imposes sustained tensile stress on the outer arc of the insulation

while the extrusion process introduces molecular orientation and residual stresses into the TPI matrix. Although the VPI temperature (130 °C

72 h) is well below the glass transition temperature (about 245 °C)

sub-

segmental relaxation and amorphous structural rearrangement occurred

leading to free volume redistribution and residual stress relaxation. The constraint of the metallic conductor further restricts dimensional recovery

thereby promoting microcrack initiation. Second

low-molecular-weight species from the epoxy system

particularly the curing agent HY918 and the accelerator DY073-1

permeate into the amorphous regions

of TPI

causing plasticization

swelling

and enhanced segmental mobility

which significantly reduces the local resistance to yielding and crack propagation. Under sustained bending stress

this results in a typical environmental stress cracking (ESC) acceleration effect. Third

the substantial mismatch in the coefficients of thermal expansion between the TPI and the metallic conductor generates considerable thermal stresses during the VPI thermal cycle

further exacerbating the cracking risk. Based on the mechanistic understanding

a protective strategy was implemented by wrapping the TPI wires with a thermal insulation layer prior to VPI. The protected specimens successfully passed the vacuum Paschen test

confirming that this approach effectively prevented resin permeation and thermally induced structural transitions in TPI insulation. This strategy provides a reference for the failures of quench diagnostic wires in similar superconducting magnets.

关键词

Keywords

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