Cross-Linking Reaction under a Stress and Temperature Field: Effect on Time-Dependent Rheological Behavior during Thermosetting Polymer Processing AITranslate

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DOI：https://doi.org/10.1021/acsapm.9b00432

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In most equipment that processes thermosetting polymers, a material is subjected to a large stress field while the cross-linking reaction is taking place. It is therefore vital to understand how the time-dependent rheology of a cross-linking thermosetting system gets affected by the stress field. In this work, an effect of the creep flow field on a model thermosetting system of epoxy resin undergoing chemical cross-linking reaction has been studied at different temperatures. It has been observed that creep curves obtained at different reaction times and stresses do not follow the Boltzmann superposition principle in the real-time domain. However, when transformed from the real-time domain to the effective time domain, the cross-linking system is observed to validate the Boltzmann superposition principle leading to time–reaction time–temperature–stress superposition. In the effective time domain, the real-time is suitably normalized by the time-dependent relaxation time, which shows a power law dependence, so that the material properties become invariant of the effective time. The corresponding power law exponent is observed to decrease with an increase in stress, suggesting stress to have a retarding effect on the evolution of relaxation time during the cross-linking reaction. Instead, the power law coefficient increases with an increase in temperature, indicating an enhanced time rate of evolution of the relaxation time at higher temperatures. This behavior also indicates that while stress impedes or temperature accelerates the evolution of mean retardation/relaxation time; it does not affect the shape of retardation/relaxation time spectra of the reacting system in the pregel regime. This work provides important insights into how the time-dependent rheology of thermosetting systems is affected by the stress and temperature fields and, therefore, may help in designing better processing equipment.