Novel test method to measure time-cure superposition shift factors of filled-thermoset under isocure testing conditions

TL;DR

This paper introduces a new method to accurately measure time-cure superposition shift factors under isocure conditions, improving the reliability of the theoretical model.

Contribution

A novel test method is proposed to measure TCS shift factors under isocure conditions, solving issues with existing techniques.

Findings

The proposed method produces excellent overlaps in master curves across the entire curing range.

The validity of time-cure superposition and applicability of time–temperature superposition to partially-cured specimens are confirmed.

The method is implemented successfully for an epoxy-based molding compound.

Abstract

Despite the solid theoretical foundation of time-cure superposition, the time-cure superposition (TCS) shift factors reported in the literature do not support the theory very well. The discrepancy stems from the non-isocure test conditions used in the tests. This study proposes a novel method to eliminate the inherent problems of existing techniques to measure the TCS shift factors, i.e., to measure them under isocure test conditions. The proposed method optimizes a test procedure while offering sufficient relaxation but producing no or negligible additional curing during testing. Optimization requires a complete understanding of curing behavior not only in the chemically-controlled domain but also in the diffusion-controlled domain. The method is implemented for an epoxy-based molding compound. Portions of the storage master curves are obtained at four partially-cured states \documentclass[12pt]{minimal}
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\begin{document}$$(p = 0.6, 0.7, 0.8, 0.9)$$\end{document}(p=0.6,0.7,0.8,0.9), and they are normalized by the corresponding equilibrium modulus. The normalized curves are subsequently shifted to determine the TCS shift factors using the master curve of fully-cured specimen as a reference. The results show excellent overlaps over the entire curing range after shifting, corroborating that the proposed method is accurate and effective. Validity of the time-cure superposition and applicability of the time–temperature superposition to partially-cured specimens are also confirmed using the test data used to determine the TCS shift factors as well as additional test data.

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