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

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 <i>non</i>-<i>isocure</i> 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 <i>isocure</i> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo> <mml:mi>p</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.6</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0.7</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0.8</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0.9</mml:mn> <mml:mo>)</mml:mo></mml:mrow> </mml:math> , 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.