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Matrix-dominated physical properties of a carbon-fiber-reinforced epoxy composite and a neat epoxy resin have been found to be affected by annealing in nitrogen below the glass transition temperature Tg. Postcured (±45)2s specimens of Thornel 300 carbon-fiber/Fiberite 934 epoxy as well as neat Fiberite 934 epoxy resin were quenched from above Tg and given a sub-Tg annealing at 140°C for times up to 105 min. The damping behavior, stress-relaxation rate, and the enthalpic state of the matrix material were found to decrease as functions of annealing time. No weight loss was observed during annealing. The time-dependent change in physical properties is explained on the basis of free-volume collapse that is related to the physical aging of the nonequilibrium glassy network of epoxy. Physical aging in network epoxy has been demonstrated to be a thermoreversible process. The kinetics of this aging process were monitored by differential scanning calorimetry, stress-relaxation tests, and dynamic mechanical analysis.
Stanford University, Stanford, CA
Stock #: CTR10870J